Early sign of HB failure?

My '89 is making a noise like someone cracking knuckles, a clacking from the belt side of the engine, down low. Doesn't make he noise when the belt is off. I've ruled out the accessories with listening with long screwdriver.

Are there marks on the HB to see if it has slipped? (like a Corvair HB?)

check the rubber in front (easy) and back (not so much) for cracks or missing pieces in the rubber on the crankshaft (harmonic) balancer. if found that is probably the problem. i would install a new crank sensor at the same time. this problem will not throw a "code".

but - are you losing power occasionally when driving or bad idle? those are usual symptoms of the balancer/sensor. since you stated the sound is down low on the engine, it's possible the a/c compressor could be a problem. here's a way to remove the a/c by a smaller serpentine belt: http://reattaowner.com/roj/component/content/article/95-accessories/heat-a-ac-system/113-belt-to-bypass-the-ac-compressor

i'd rule out the balancer, then the a/c compressor as the problem before moving on. possible water pump/timing chain issue(?).

hope this helps.