There is a black cover you can remove on the front of the bellhousing and look inside (with a mirror, super awkward). The flex plate is somewhat open to the enviroment so you could actually have a small pebble rattling around in there. I replaced the cover's gasket thing on mine a while ago which had disintgrated, the bellhousing was full of crud.
Or the flex plate could be cracked. I just replaced mine as PM not too long ago while the transmission was pulled to do the torque converter seal. Did the rear main seal as PM too. Wasn't cheap the flex plate is about $300. You should do the rear main seal, transfer shifter seat and starter contacts/plunger as PM while the transmission is pulled.