If I have my car in park or nutral it will idle with no problems, no strange sounds or anything like that. When I put it in gear it will idle fine for about 30-60 secs and then I hear a kinda of grinding noise and feel a vibration on the floor. As soon as I put it back into park it stops, put it in gear, 30 secs later it starts again. If I pull away it stops almost right away too.
Its been doing this for sometime now. Some days it wont do it at all but others it will start as soon as I start the car (doesn't matter if the engine is hot or cold). I changed the tranny fluid and filter but that didn't help. I did notice when I changed the fluid that the inside of the pan had rub/scuff marks on it.
Any ideas what it is? or is it just the filter vibrating against the pan?

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'89 IROC, custom 600cfm Holley carb, Street Dominator Intake, Flowtech headers, no cat, Flowmasters, no emissions (smog pump, charcoal fillter etc), remote starter solenoid, push button start, getto sound system, none leaking sunroof