The crankshaft balancer (pulley) is TWO PIECE - the outer ring which drives the belt is likely slipping off the inner hub..
Visual inspection / *hands-on* evaluation will be required.
Good luck!
IF this is determined to be the case - balancer replacement is the only fix.
Drive belt will need to be replaced as well - it HAS BEEN compromised and will not track properly - will likely run off pulleys if reused.
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IF this is determined to be the case - balancer replacement is the only fix. Drive belt will need to be replaced as well - it HAS BEEN compromised and will not track properly - will likely run off pulleys if reused.