I drive a 2002 Escalade AWD, and when the car is in gear with the AC running I hear and feel a loud vibrating noise coming from under the hood in the area of the AC compressor. When the AC is off, or when the car is in park, the noise stops and the car runs great! I took it to my mechanic of choice, and he replaced both belt tensioners and the AC belt. However, I still have the same problem, and the sound appears to be coming from the area of the lower tensioner. This problem seemed to come out of nowhere last week, but the AC still runs fine while cooling the car in the 100 degree Kansas heat even while making the loud vibrating noise (the sound is similar to the sound of sticking a pencil in a fan) but the idle drops and the car runs rough. The mechanic also checked the fan clutch which he found to be OK. I have taken the car back to the mechanic twice now, so please help me figure out what is going on. Thanks.