I've been a little worried about an engine "wobble" for lack of a better term. It wasn't misfiring, but there was a noticeable vibration at certain RPM's. Like at about 1100 RPM at idle with the AC on. I checked everything...there is a fairly new crankshaft balancer/pulley on it. Everything was tight. Was about to start replacing the engine mount rubbers (which wouldn't hurt anything). Well, I've been ignoring an intermittent belt squeal...newish belts have broken in and needed tightening. It only happens at about 3200 RPM so I'd make a mental note then forget it because I rarely drive the engine that fast. I finally got around to getting the belts all tight and whaddya know but the wobble is gone, too. It was the water pump belt that did it. Anyway, lesson learned!