Vibration

I grounded the plane today because the vibration sounded like a knock. Mag drops, static rpm, oil pressure all normal. But there is a definite knock sound that seems to be on one cylinder. A local A&P is looking it over this afternoon. I'm not going to fly it until we get this figured out.

Larry
N99340

My airplane is 415C with a C85 with an O200 crank. Engine was rebuilt in 2006 and torn down for inspection and new bottom end in 2009 (don't ask). It has served me faithfully for 1200 hours since the rebuild. In the past few months, I have noticed a random vibration. This kind of has the feel of things touching and vibrating against each other, but there are no signs of it anywhere. We just put in new motor mount bushings, no difference. The engine seems to be performing like it always has, I climb out alone with full fuel at 1000 fpm and I cruise at 110 mph. The vibration seems worse at some times than others. Everything under the cowl is tight and where it's supposed to be. Also, I'm not even sure if it's just my expectations rather than a real problem, but I think I'm pretty sensitive tot he old girl, as I have 1250 hours in her.

I know many people have dealt with vibration, a truly difficult problem to solve, and wonder what issues you all have uncovered when it comes to this? Thanks!

Larry
N99340