I have a 2010 X5M with 11,700 miles. Just purchased preowned. Bringing it in on Friday but wanted to get some opinions on a noise I am hearing when the AC is on which is quite frequently. Details as follows:

1) Noise is not normal as I have had X5's previously (not a M).
2) Noise can best be described as a high(er) than normal hum or buzz that is present at or around idle.
3) Noise ceases when on the gas above idle.
4) Noise is gone when I turn off the air totally or when I push the AC button i.e. noise turns off when AC is off.

So, is it the belt? Compressor? Overcharged system? Harmonic imbalance?

TIA for assistance.