You had the transmission out. Something scraping in the torque/flexplate area intermittently? It might be hard to localise from above with a stethoscope.

I have a stupid problem of this nature with an other brand car. As long as driven daily no issue. Park it a few days and it reocurrs. Park it more than a week and initially you think it should be taken apart for a look. I make sure it stays in service daily.