[Shotimes] Clicking Noise

[R and S]

I have an extra engine in the garage that turns fine, but produces a 
clicking noise from one of the heads.  I took the valve cover off to 
discover that when turning the engine the timing chain slacks then snaps 
back into position and clicks against the chain guard.  Anyone have 
ideas on what that could be?  I can't tell if the problem originates 
from the top or bottom end.  The chain feels tight to the touch.  What 
would cause the momentary slack in it's revolution?  Thanks..

- Rich
89 Titanium