Have a vibration between approx. 2000rpm to approx. 3000rpm. Smooth in all other rev ranges. Parked in driveway in Park, or cruising down the hyway. Alternator is new. As is, harmonic balancer. Other than the alternator and balancer, both of which I replaced, the other accessories appear to be used, most likely from the same engine pre-rebuild. Took the belt off and vibes stopped.Could be a belt tensioner - my first suspect. Is there a short cut to checking the other front of engine accessories? Motor mounts appear to be solid. Engine rebuilt within 10000 miles. Any help appreciated. Thanks

With the belt attached & the engine running, touch a long metallic object to each accessory. Listen for a "weird" noise.

WARNING!!!!!
Do not stick anything in or near the belt while the engine is running.

If the belt is still off. Spin each pull by hand & see if there is a gritty feel to it.

No loose clothing. No long hair and no jewelery. Ever seen a cat that got stuck in the fan shroud after starting the engine? A metal fan at that. Gory AF.

Less than $5.

Why didn't I think of that? Thanks.