I have a 95 Tahoe with the an R4 compressor. After the AC has been running a few minutes I get a growling rattle sound. I turn the AC off and it's gone. It is most noticeable coming from the alternator. Even using a stethescope it is loudest at the alternator. Pressures appear normal. I suspect it's my aging compressor going out making the noise. I thought it was being caused by an old tensioner so I changed that without making any difference. The alternator and AC both spin freely by hand with the belt removed.