[PAman]

Could be the flexplate has slipped on the crank and/or is lose.(this has the driven gear that mates w/starter and is the interface between the crankshaft and the torque converter...roughly equivalent to a flywheel) Cheap to buy the new parts, but the engine will have to pulled. I changed mine when I put another engine in it...and it showed signs of sippage at the crank. I understand GM has changed the crank bolt torque procedure to a torque-angle procedure for this reason. Difficult to diagnose without pulling the engine, but it will get worse. Possibly, the torque converter inspection cover can be removed and, with a flashlight, you may be able to see if the flexplate-to-crank bolts have loosened. While you're there, check the flexplate-to-torque converter bolts .