Help! The AC compressor on my 86 Caballero has developed a loud growl/low frequency vibration at idle speed only. Above idle and there is no noise. The system works fine. A new (not reconditioned) compressor was installed in 2000 at the same time the system was converted to R-134. I took it to the installer and he said he thought it needed a new belt. Changed the belt, but did not help. I also snugged all mounting bolts, etc. Installer said he does not know what the problem happens to be. Any ideas??