Car rumbling at idle

1992 Chevy Cavalier, about 120,000 miles. A few months ago my car has started to rumble and vibrate while idling in drive. It only occurs while I’m in drive and completely stopped. If I change to neutral or park, it immediately goes away. Also, if I’m in drive, keep my foot on the brake and give it a little gas, the vibration goes away. I’ve changed the spark plugs, had the throttle body cleaned (not very thoroughly) and my mechanic said my engine mounts look fine and to not worry too much about it. Is this a problem I can ignore as an inconvenience or should I put more effort into getting this fixed?

It would appear the idle is set too low.

What is the idle rpm in PARK?

Any way to figure this out with a car without a tachometer?