[infinity89]

I have a 1989 Chevrolet Suburban and it does the same thing but only when in park or neutral or if I am slowing down for a stop light or something of that nature. My dad is a GM Master Mechanic and he told me it coluld 1 of two things.....loose torque converter bolt(flywheel to torque conv.) or it coluld be the crank.... hope this helps.....