93 Saturn sc2 automatic. Have a stumper here.

When the fan for the a/c (or heat) is running AND car is in drive or reverse, there''s a loud "clanking" sound. It doesn''t appear to be rhythmic. When the a/c fan is off, no noise at all. When a/c fan is on and in park or neutral, no noise. In other words, the noise is only there when the a/c fan is on and the car is in drive or reverse.

Also, when the noise is occuring, the engine visually "shakes" more than when the noise is not happening.

No noticeable difference in how the engine runs and the a/c cools great.

Any ideas?

This is from your belt tensioner knocking against itself. Replace your belt first. This might fix the problem.