Whining sound when ac is on

[Noquacks]

People,

2009 BMW 328i, original owner, 70,000 miles, manual trans. When I shift gears with ac on there is a whining sound that increases with pitch matching the gears I am shifting. The sound matched 100% with the rpm's. But as soon as I turn off the ac button the whining stops.

AC is ice cold. Could it be a bad serpentine belt? Compressor starting to go bad (bearings, etc)? Maybe the spring tensioner on the serpentine belt?

Thanks, People.

[Cusser]

Could it be a bad serpentine belt? Compressor starting to go bad (bearings, etc)? Maybe the spring tensioner on the serpentine belt?

If my vehicle, I'd pull the serpentine belt and then spin the tensioner pulley by hand, and feel/listen for an issue with that.

And how old id your serpentine belt?