ONE TIME I parked and when out into PARK, heard grinding noise; 2nd time I parked no such noise

I had trans completely rebuilt & trans mount replaced 3/30-4/6/12. Car drove/handled beautifully. Then yesterday I had to move car a short distance (24') for gardeners and when I put it in PARK I heard a metallic grinding sound that did not cease with application of parking brake until ignition turned off. When I moved car again, drove around block 10-25 mph (commmunity speed limit) and pulled into my driveway, car went fine and went INTO PARK without any grinding noise or other problem. What could have happened? Car has been running absolutely wonderfully. Is it dangerous to drive now?