There was a GM bulletin back in the day to replace the clutch pulley on the compressor. The original is to light and will cause a vibration. I had 1987 GMC that I bought new that had that problem and had it replaced under warranty at the time. I took away about 90% of the vibration. That pulley is no longer available from GM. On my 1987 that I am restoring now I purchased a new GM compressor and it has the newer pulley on it. If I remember the original pulley was very lightweight and kind of tinny. The new pulley was heavier and had more meat on it. The compressors you are buying may have the lightweight pulley on it. Also it is important that you make the belt pretty tight. It should not bounce. If you are not used to tightening V belts you may not be making it tight enough.
#88-413-1B: MOAN NOISE WITH A/C ON INSTALL NEW COMPRESSOR PULLEY - (Apr 16, 1989)
Model and Year: 1985-88 LIGHT DUTY TRUCKS AND VANS WITH R-4 A/C COMPRESSOR AND WITHOUT SERPENTINE BELT DRIVE
TO: ALL CHEVROLET DEALERS
Chevrolet Dealer Service Bulletin 88-176-1 B dated March, 1988 is being cancelled and superseded by this bulletin to include "P" model trucks. All copies of 88-176-1 B should be discarded.
An A/C moan on vehicles equipped with one of the engines listed below can be addressed by replacing the A/C compressor pulley.
NOTICE: Prior to ordering the new pulley, visual verification should be made that the vehicle is equipped with the R-4 compressor and is not serpentine belt driven.
YEAR MODELS ENGINES ---- ------ -------- 1985 C/K 4.3L 5.0L 5.7L 6.2L Diesel M 4.3L G 4.3L 5.0L 5.7L P 4.8L 5.7L
1986 C/K/P 4.3L 5.0L 5.7L M 4.3L G 4.3L 5.0L 5.7L
1987 R/V/P 4.3L 5.0L 5.7L G 5.7L (LT9 ONLY)
1988 R/V/P 5.0L 5.7L G 5.7L (LT9 ONLY)
The new pulley, P/N 6552402, is a single "V" groove high inertia pulley, which is heavier than the original "V" groove pulley