Warbling whistling sound from air blowers in cold weather

Hi

E39 530i 2001 180k miles

For the last few winters I have had a strange warbling whistling noise from my air blowers on cold days (below about 5 degrees C/40 degrees F). Have asked my mechanic to look into it but he's baffled - though to his credit he has advised against expense of stripping out dashboard/fascia to investigate. Key facts:

1. Sounds a bit like someone (or a bird!) doing a sort of oscillating whistling - a fast tweeting sound
2. Gets louder/quieter according to the speed of the air blowers, and stops completely when blowers are turned off
3. It doesn't stop when the cabin or the engine have warmed up (in other words it seems to be related to external air temperature only)
4. Happens whether air con is selected or not
5. Gets quieter when I turn the steering wheel to extremes - so is the root of the problem somewhere near the front wheels/steering mechanism?
6. First noticed it after I replaced the FSU (using original BMW part), but this may be coincidence. Mechanic said that replacement FSUs sometimes don't seat in very well so he reinforced the seal with some black gooey beading. But if it is the FSU, what about 3 and 5 above?
7. Blowers/air con performing perfectly well in all other ways

This is something that has really been annoying me for some time so any help would be gratefully received!

Thanks

Maybe your blower motor is on the way out.

Maybe a plastic part is not seated properly, or has deformed in time, and in cold weather it's somehow contracted, creating an unwanted "vortex' where 2 pieces should mate, and now they're leaking?

Try to open the hood and double check both boots that go from the cabin filter into the firewall. I know one boot on my car lost one of the claws (embrittlement), and on another occasion, the other one wasn't mounted properly fogging up the passenger window every time I was in a car wash. Nobody could figure out the problem. Made also a weird sound.


Part #7 in this pic:

For the last few winters I have had a strange warbling whistling noise from my air blowers on cold days

Likely not the blower motor; if it was, the bearing would have seized by now.