This is going to sound odd but when I reboot my 14 Pro Max, I get a "thunk" sound from the speaker when the SIM PIN entry screen pops up. At first I wondered if it was some sort of startup chime but I've not enabled the shutdown/startup sounds in Settings. It sounds like it's deliberate rather than some sort of artefact. Never had it on any other iPhone - just the new 14 Pro Max.