This article is from the Chrysler-Plymouth-Dodge FAQ, by Dr. David Zatz with numerous contributions by others.
From: firstname.lastname@example.org (gary cristadoro) writes: A constant
knocking/metallic clanking which increased with engine rpms and got louder
with engine speed on a Jeep turned out to be the vibration dampener
(harmonic balancer) bolted to the driveshaft. It drives via belt all
accessories. You can visually inspect the rubber insulator between the yoke
and counterbalance pulley; if a severe shift has taken place (the two parts
are misaligned and protrude), the fan belt is not aligned with other