Hello everyone!

I'll get straight to the point; I have a strange issue in my 2006 IS250 auto transmission. Very often when the engine is at idle (no matter the gear mode it's in) I can hear this strange whirring or zooming noise inside the cabin, almost as if there's a phone vibrating inside the dashboard. However it is intermittent and very unpredictable, at first I thought it might have been the AC compressor giving out, but I can hear the sound faintly when hanging over the engine bay yet it is not synced to the AC compressor turning on and off. But the weird thing is, maybe it is coincidental but when I turn the AC off the sound seems to stop. It really only happens at idle around 700/800 something RPMs. Whenever I try to record it, it ofcourse does not happen and the times I have been able to record it you can barely even hear it over the engine noise.. I can even somewhat feel it in the floorboard and pedals when it happens. It seems like it had only started to show up after I had replaced the water pump. I reused the old serpentine belt (I know, should've installed a new one) could that be the issue, do I need a new serpentine belt? Water pump is fine, Toyota OEM parts and no leaks whatsoever after about 6 months.

I hope someone can help me out here as it actually is driving me crazy. Maybe it is related to the HVAC system inside the dash but due to it being related to engine RPMs I am just not sure..

Many thanks to whoever is able to bring me some new insights!