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I’m not sure why everyone is only reporting clamshell mode. I run my 2017 MBP open with a 3440x1440 monitor all day and it’s completely silent and cool to the touch even with many programs open unless I turn on my Windows VM or run some serious scripts locally.
 
This is mine i9 2.3ghz , 5500m , 16gb, 1tb.
Using 2012 imac as monitor via thunderbolt 2-3 adapter. Both imac and macbook screen resolutions/scaling set to 'Default'. Fans always around 2600rpm on idle. Very annoying. Also tried 4k LG monitor with usb-c hdmi adapter, also usb-c mini dp adapter, same result. On small load, fans go crazy and after the load, it takes 10 mins to fans to go back to 2600rpm. They never return to 1800rpm, which is normal idle fan speed.I suspect it's not about cpu temperatures, but it's about thunderbolt or Platform controller hub die temperatures. Also looks like Windowserver process is always using some cpu.
When not connected to external monitor, everything seems normal. Fans around 1800rpm on idle.
 

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These stories seem like defects of some sort. I guess I’ll wait a while before considering buying so that they can work put the bugs.
 
I suspect it's not about cpu temperatures, but it's about thunderbolt or Platform controller hub die temperatures.

It's the GPU running hot. The 5500m runs a way hot than the old 560 or even Vega.

Mind that the same heat pipe goes trough GPU and CPU. When connecting an external Display, the GPU start to work harder, it's pretty obvious that it will become hot.

Please look at this picture. There's only one heat pipe!

Screenshot 2019-12-08 at 09.47.28.png
 
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Just an update; I've made a clean installation of Catalina, same result.
I've installed latest Catalina 10.15.2 Beta 4 , same result. (noisy when connected to external monitor)
 
Just an update; I've made a clean installation of Catalina, same result.
I've installed latest Catalina 10.15.2 Beta 4 , same result. (noisy when connected to external monitor)

Not sure what is happening with your device - I can do some light work for hours (web browsing, multiple tabs) on external 4k monitor (both screens are up -not clamshell mode) and cannot hear fans (no, I don't have hearing problems ;)). Temp is approx 10-15 C higher (50-55deg C with external connected vs 35-40 deg C without) but fans are at min.

When I play Tomb Rider at high settings fans are noisy like crazy but that's another story ..
 
Just an update; I've made a clean installation of Catalina, same result.
I've installed latest Catalina 10.15.2 Beta 4 , same result. (noisy when connected to external monitor)

Just curious, did you try to connect your monitor to the other side of TB3 ports (other bus)?

I am waiting for mine (i9, 32GB RAM, 5500 8GB) and am planning to use it with two 24" 4K LG Ultrafine monitors.

I am currently using a 34" LG on a Blackmagic eGPU 580 via HDMI with 2018 Mac mini on Catalina, System is dead silent while using FCPX
 
It's the GPU running hot. The 5500m runs a way hot than the old 560 or even Vega.

Mind that the same heat pipe goes trough GPU and CPU. When connecting an external Display, the GPU start to work harder, it's pretty obvious that it will become hot.

Please look at this picture. There's only one heat pipe!

View attachment 881475
The are multiple heat pipes the same as on previous mac but they look bigger bigger and run to 35% bigger heat sinks.

See the iFixit tear down for details.

https://www.ifixit.com/Teardown/MacBook+Pro+16-Inch+2019+Teardown/128106 Steps 7 and later have the details.
 
It's the GPU running hot. The 5500m runs a way hot than the old 560 or even Vega.

Mind that the same heat pipe goes trough GPU and CPU. When connecting an external Display, the GPU start to work harder, it's pretty obvious that it will become hot.

I don’t believe the Navi GPU runs hotter than Vega. Most Navi reviews (especially for the desktop 5700/5700XT) show that Navi uses less power than Vega, which is to be expected, given that Navi is built on TSMC 7nm.

My understanding is that Apple configures the GPU TDP to approximately 35W in all cases across Polaris, Vega, and Navi to stay within the machine’s overall power and cooling budget.

I recently sold my external monitor because the 16” internal display is so big and pretty that I use it exclusively.
 
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OP:

Why don't you disconnect both displays, and then try them "one-at-a-time" to see what results you get?
 
Not sure what is happening with your device - I can do some light work for hours (web browsing, multiple tabs) on external 4k monitor (both screens are up -not clamshell mode) and cannot hear fans (no, I don't have hearing problems ;)). Temp is approx 10-15 C higher (50-55deg C with external connected vs 35-40 deg C without) but fans are at min.

When I play Tomb Rider at high settings fans are noisy like crazy but that's another story ..
Do you mind sharing sensors readout screenshot from " istat menus " app?
As You can see from my screenshot, my monitor is connected to left Thunderbolt and it is 55 C on idle. Fans are running 2400rpm, which is audible on a silent room.
[automerge]1575910127[/automerge]
Just curious, did you try to connect your monitor to the other side of TB3 ports (other bus)?

I am waiting for mine (i9, 32GB RAM, 5500 8GB) and am planning to use it with two 24" 4K LG Ultrafine monitors.

I am currently using a 34" LG on a Blackmagic eGPU 580 via HDMI with 2018 Mac mini on Catalina, System is dead silent while using FCPX

I've tried all 4 ports. Charger on and off. LG 4k 650 monitor, with USB-c to hdmi adapter, usb-c to Displaylink adapter, iMac as Thunderbolt monitor. Results are same, always noisy. Contrary, I've tried a 2019 13" Macbook pro, and it is quiet, fans running always at minimum even if connected to a monitor.
I guess I'm gonna return my Macbook 16" and get a new iMac + Macbook Air.
 

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Do you mind sharing sensors readout screenshot from " istat menus " app?
As You can see from my screenshot, my monitor is connected to left Thunderbolt and it is 55 C on idle. Fans are running 2400rpm, which is audible on a silent room.

I did already - see here:
idle: https://forums.macrumors.com/threads/16-is-hot-noisy-with-an-external-monitor.2211747/post-28028111

lite web browsing, YouTube (1080p) on 4k scaled to 1080p:
https://forums.macrumors.com/threads/16-is-hot-noisy-with-an-external-monitor.2211747/post-28030938
https://forums.macrumors.com/threads/16-mbp-i7-or-i9.2212684/post-28036911
 
Did you notice that there's no heat pipe on GDDR?

I did notice the memory is not heat pipped, but this does not surprise me. But the dGPU module is definitely heat piped as shown Step 7's 3rd image.

FWIW, the Dell XPS 15 I have does not have a heat pipe to it's GDDR
 
Do you mind sharing sensors readout screenshot from " istat menus " app?
As You can see from my screenshot, my monitor is connected to left Thunderbolt and it is 55 C on idle. Fans are running 2400rpm, which is audible on a silent room.
[automerge]1575910127[/automerge]


I've tried all 4 ports. Charger on and off. LG 4k 650 monitor, with USB-c to hdmi adapter, usb-c to Displaylink adapter, iMac as Thunderbolt monitor. Results are same, always noisy. Contrary, I've tried a 2019 13" Macbook pro, and it is quiet, fans running always at minimum even if connected to a monitor.
I guess I'm gonna return my Macbook 16" and get a new iMac + Macbook Air.
I seem to be getting similar numbers from my machine, but I literally had to check iStatMenus to see if my fans were on they are so quiet. I do hear the fans when they come on full blast, but this is a far cry from that. How does the noise level change if you use iStatMenus to manually adjust the fans to medium or high?
 
I seem to be getting similar numbers from my machine, but I literally had to check iStatMenus to see if my fans were on they are so quiet. I do hear the fans when they come on full blast, but this is a far cry from that. How does the noise level change if you use iStatMenus to manually adjust the fans to medium or high?
I couldn't change the speed of the fans with istat menus. I used 'mac fan control' app, At minimum, around 1800rpm, fans are very quiet. Around 2400rpm, I can hear them well and they disturb me. When connected to the monitor, it likes to keep the fans around 2400rpm. If I force them to stay around 1800rpm, laptop overheats and throttles itself until it is unusable.
 
Very interesting results.
You get 35 C on CPU when not connected to the monitor. I get 50 C.
Two factors in my opinions - ambient temperature (relatively fresh in my case ~18deg C) and initial indexing/syncing (Photos took nearly two weeks to finish - I have big >1,3T photos library). If you withdrew those then something is wrong with your machine.

EDIT
or the whole CPU binning discussion is valid and your 2.3 i9 thermal output is not as good as mine 2.4 i9
 
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You are running at a true retina resolution there, i.e. 2x exactly. It would be interesting to see if one of the scaled resolutions gives you the same result. Like 4K at “looks like 2560x1440“ (not the low resolution one) where it then renders at a much higher resolutionand scaled down.
 
You are running at a true retina resolution there, i.e. 2x exactly. It would be interesting to see if one of the scaled resolutions gives you the same result. Like 4K at “looks like 2560x1440“ (not the low resolution one) where it then renders at a much higher resolutionand scaled down.
I'll do the test and report back later
 
ok - see below.
I'm running external 4K with scaled (2560x1440) resolution. Playing YouTube 1080p videos in safari.
Running on battery.
I will let it play - connect the charger as well to see if it impacts my readings - will post update after a while.
Screenshot 2019-12-10 at 11.55.11.pngScreenshot 2019-12-10 at 11.55.21.png
 
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