Strange noise after engine "Dieseled"

I have a small block 355 engine that was rebuilt about 1500 miles ago. It has headers, a medium cam, forged pistons/rods. It has double hump heads, 650 CFM holley etc. Saturday, I turned off the ignition and the engine kept running for a couple of seconds, kind of dieseled. I heard a strange clunk noise, it sound like something from the back of the engine.

I restarted the engine and when I did, I noticed the starter sounded a little odd, like it didn't mesh just right. Then at about two different rpm's, around 1000 and again around 2000, there is a vibration that was not there before. After starting a couple more times, the starter engages but does not turn over the engine. I can tell the starter has power, because when you place in the start position, it dims the lights.

Do you think something could have happened to the flex plate? Broke loose maybe?

broke or loose, yep, the engine dieseling might have caused it.

id be checking to see if i could turn the motor over by hand.

Well, starter may be part of problem, too.I've seen both bendixes
& flywheel/flexplate ring gear teeth messed up.You did not ment-ion whether starter still turns over or not,so I imagine it does.But,
not meshing at all.Hope I did not overstate the obvious or mis-
understand and put a shoe store in my mouth.Hope I helped.

or on another thought, if it is jammed on the flexplate, you should be able to turn the motor backwards then it should be able to go forwards until that spot, just a thought