GarageBand: MYSTERY 'Static' w/Blue Snowball & 10.11.6

[Jodeo]

Mystery, Random ‘Static’ on Vocal Tracks in GarageBand 6.0.5:

AUDIO SAMPLE: https://bit.ly/2tVuNDy


SET UP: iMac 2007, 6GB Ram, 2TB HDD
OS: Mac 10.11.6 - El Capitan (Not an issue in Yosemite or earlier)
MIC: Blue Snowball USB (I've used two different USB cables + all 3 ports on the iMac; no USB hub)
WIFI: OFF
BLUETOOTH: OFF
OTHER CABLES: UNPLUGGED (only mic & Mac USB keyboard; no MIDI, etc. Also tried w/o keyboard).
APPS OPEN: NO BACKGROUND APPS RUNNING


NOTES:

1. Noise began immediately ONLY after upgrading to Mac OS 10.11; never occurred before that (Yosemite, Mavericks).
2. Happens every time I use GB - regardless of time of day, weather, etc.
3. Not related to any movement of mouse or keyboard or observable events on the iMac.
4. Since upgrading to El Capitan GarageBand '11 has otherwise worked fine - just this mystery static that occurs only on vocal tracks.
5. Noise/static doesn't occur on MIDI tracks or audio loop tracks).
6. When either recording vocals or practicing with playback only, the static noise occurs at random and in no discernible interval.
7. I have tested other GB vocal track settings including dry (“No Effects”). It occurs in a complete project as well as in new projects with no other tracks. In other words, this does not appear to be related to performance or other plug-ins available (but not used) in GarageBand 11.
8. Occurs on all three settings of the switch on the back of the microphone.
9. No new software/drivers to download from BlueDesigns.com (Blue Snowball website).
10. No clues from Activity Monitor.

Before suggesting downgrading to Yosemite or switching from this version of GB to a later version or different app (all planned for a new 2017 iMac, btw - going to Logic) I'd like to see if there's anything else I can try.

What else should I look for or test?


AUDIO SAMPLE: https://bit.ly/2tVuNDy


Thank you.