Rapid Ticking, not cooking

I used the MW last night, cooked first item no problem. I put in second item and could here a rapid ticking behind the control panel intermittently. When time was up I took item out and it was still frozen, WTH? I put it back in, this time no ticking and item cooked as planned. I also warmed up a couple other things with no problem. I don’t really want to wait till it is dead, any ideas?