I took my car to a mechanic for this but they couldn't get the sound to occur, so they charged me $100 for looking at it and didn't diagnose the problem. It only happens at random, and it seems to almost always occur when I get home from work late at night. I can go shopping all day and not have it happen once... But what it started out as wasa loud noise that seemed to be coming from the engine that only happened once car was completely turned off. And now it is a harsh loud vibration. I did get a recording of it today finally and will try to figure out how to attach it. Idk that it has anything to do with it, but this all started to happen after I got my oil changed. I was thinking it was the engine but now I'm worried it's something else. I have two little kids so I don't want to drive the car around if it is unsafe. It also only happens periodically not every time I shut it off.

What could be problem ...thanks