With the hotter weather out here, I've had to use my A/C at times. I noticed that there is a little bit more of a high pitched sound when the "snowflake" button gets depressed versus when just the fan is running. Is this the norm for a 2011 MC S? I don't have the climate control upgrade, just the stock a/c

 

I noticed mine kicks in and has a loud hum and then kind of goes back to a normal fan noise...not sure if that's what you are describing. I have a 2012 MCS and no auto climate either.

 

Yeah, maybe what you are describing....but mine ALMOST sounds like ignition hum on a lousy a.m. radio....fluctuating in pitch with engine revs..

 

I have an 08 S, and there is def. more noise while the AC compressor is running.

 

Sounds like a rolling tin can with coins in it!!