Ok ...My a/c is working flawlessly even in the mid 90's heat. I have owned the truck for a few weeks but have noticed this since I bought it . When I run the air i hear this weird ..whirring/humming sound that changes with engine rpm but is most noticeable when in o/drive/low rpm situations. If i tun the air off it immediatly stops the noise. I have listened to the compressor and it seems to make no weird noises. This sound seems to radiate from the trans tunnel area. BEATS ME! Everything is normal other than the noise. I know this has to be abnormal sound. Itds actually very noticealble. Thanks for reading and any responses. Any help would be appreciated. Here is the run down on the truck.......99 4x4 F-150 Lariat Supercab Flare side..5.4L...Auto. 104,000miles. Runs like a new one!

 

Im guessing no-one has any ideas?

 

I have the same noise on my 98 Lariat SC. AC works fine. All I can guess is a vibration somewhere.

 

I know that it drives me insane. It definitly only does it while running the ac. Sounds like right under the trans tunnel. Kick the a/c off in 5 seconds it stops. Its very annoying. I just cant pin point it. Thanks for replying. Maby someone will chime in with a anwser or some opinions.