Yesterday afternoon, I was in cruise flight in my M20F. Full throttle, 2500 RPM, leaned to 100 degrees ROP. My feet were resting on the rudder pedals, and I suddenly started to feel a slight vibration that was not there before. Then, I could hear the engine start to sound a little slower. I was able to make it sound more normal by enriching the mixture a little. Not full rich, but richer than normal. Pulling the mixture back much at all from that point made it really rough.
I diverted and did some checks on the ground. During a run-up, the engine sounded great. At first it seemed fine at idle, but then I started to notice some roughness at idle. A slight throttle up for taxi after a couple of minutes at idle led to a little more roughness. I think it was worse with just the right mag selected.
This morning, the mechanic where I left it says it is running great. I know there is something not quite right. Any suggestions for things to have the mechanic look for?