Q: Loud grinding/grating metal noise

I started my car in the morning then went back inside for a few mins after a few minutes I heard what sounded like someone outside grinding metal which went on for a couple mins then stopped. I went back to the garage where my car was and I heard it quickly again very loud but stopped so I knew it was my car. I opened the garage door to find clear/whitish cloudy smoke in the garage I got inside my car and pressed the gas pedal slightly to rev the engine and it started griding again very loudly so I turned the car off and had it towed to my mechanic They had it for almost 2 days and nothing happened no smoke no grinding nothing, drove it around checked our the battery alternator starter and a few other things looked under the car and everything was good. I picked it up and it was great drove like nothing ever happened. The day before it all happened I had put it in reverse and it didn't roll back so I stepped on the gas and it made a big thump and then rolled back but drove home fine.

My car has 120000 miles.
My car has an automatic transmission.