Constant high pitch beep....

This is a weird one. I've done some searching on here, and it seems most likely to be related to the brake booster, but I don't have many of the symptoms.
what Ive noticed:

A 3800Hz constant beep that starts a few seconds after power on, but super rarely, and seems to happen mostly on really hot days. It sounds like it's coming from the passenger foot well (while I'm sitting in the driver's seat) Online Tone Generator - generate pure tones of any frequency listen to it here!

eventually goes away after a seemingly random amount of time, usually stops after 7-10 minutes.

Brakes work as normal No loss in braking ability, same old sucky weak regen for the first half of the pedal's travel and friction braking for the rest.

No warning lights, TPMS flashed towards the end of my most recent 45-mile commute, but I need to rule out low tire pressure first for that one. (doing that tonight)

No codes were found using a Bluetooth OBD scanner (I'm breaking out the mini VCI next. Does anyone have a link to a techsteam DL? I lost the CD that came with the cable.)

edit: I checked the tires and RL is at 30psi
Also, the pump is not lethargic when coming up to pressure and does not activate after sitting while the car is on/not moving. I waited like 10 minutes. I'll try a longer interval tomorrow