Strange noise, when AC is on
 

My 2005 Cooper S has started making a strange sound. It only happens when the AC is on. It's a weird whining. It stops about 2-3 minutes after I turn the AC off

It does this if I have the AC on heat or cool.

It was low on engine coolant also but that has been fixed and didnt stop the whining

Any ideas on what to check? There are no lights, just the sound

Thanks