I have a 70 Chevelle SB400, factory AC, my problem is this, I hear what sounds like a knock in compressor when the AC is in the off mode, when it is on it sounds fine and blows very cold air out. It is just when it is off that the offending sound is present. I had a local AC shop evacuate, refill with R-12 and ever since that time the noise has been there, it is a ugly noise sounds like a knocking valve or a timing problem but it is definitely coming from AC compressor as it goes away when turning AC on. Any ideas? anyway to check compressor oil?

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Pat"The Soundkat>^..^<"Hoffman
1970 Chevelle SS (clone)

Pat. Two things come to my mind. The drive pulley bearing or an attaching bolt.

Is the gap wide enough; are you sure the clutch is off ?

Rob

What gap would you be speaking of? And all mounting bolts are tight, it sounds like it is coming from inside the compressor, maybe valves or something, sounds like an engine knock, erratic and hollow.

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Pat"The Soundkat>^..^<"Hoffman
1970 Chevelle SS (clone)