A/C Noise Need Help

I have a 2003 CRV 4x4 and it has some problem with the A/C. There's a sort of "humming" noise whenever I turn the A/C on. The noise becomes more prominent as I hit the gas. The noise is getting louder as the car accelerates and getting quiter as it slows down. I took it to the dealer and they can't find anything. I drove my friend's CRV a while back and it didn't have that problem. Any idea? Thanks in advance for the helps.

possibly the a/c compressor clutch. did they techs at the dealer test drive it to verify you issue?