Flexplate Clatter

Dan

Don't rule out a loose transmission flexplate (driveplate, flywheel). When you say it is loudest under the car, this makes me think of the flexplate. Does the noise get loudest when you pull away from a stop sign or traffic light, then go away at higher vehicle speeds? Then it is likely the flexplate. The problem occurs when the rivets which fasten the plate to the ring gear get loose, allowing the plate to rattle on the ring gear.

Here's the how to tighten them.
1. Put front wheels on jack stands.
2. Remove sump-gearbox stiffening plate from bottom of engine oil pan/transmission bell housing.
3. Remove semi-circular (black) dust shield/plate from the front of transmission bell housing.
4. Remove two plastic protective screens from bell housing.
5. Locate rounded (vs. flat) side of rivets which rivet the starter ring gear to the flexplate. The round heads are against the rear face of the flexplate.
6. Using a roughly 4" or longer center punch, put the tip of the punch at the interface of the rivet head and flexplate. Access the rivets throught the holes where the plastics screens were.
7. Using BFH of choice, deliver several stout blows to the punch. The result is that there will be a dimple in the rivet head which squishes metal against the flexplate, producing some tension in the rivet. Looseness in the rivets is what causes the clattering.
8. Repeat steps 5-7 10 times. You will have to bump the starter with the ignition key to reach all 10 rivets.

This fixed my clatter problem. It is totally gone.

Warning: Jan Wikstrom recommended using a small sledge hammer to back-up the flexplate (on the front face of the flexplate, through the semi-circular opening). My guess is that this will prevent bending of the flexplate due to the BFH blows. I think this is a good idea, but since my spouse isn't thrilled with lying on a dirty, cold (25F) garage floor at 11:00 p.m. I didn't have a third hand to hold the sledge-dolly. I'm not sure if I bent my flexplate (even slightly) but I have had no untoward effects so far. If you can use something to absorb the BFH blows, then I think this is a good idea.