engine RUMBLE, Oil pressure fluctuating

I have a 912ULS in a RV-12. The engine is at 1400 hours. In the past 200 hours I have felt a rumble vibration centered at 4000 rpm. I have dynamically balanced the prop at .02ips and carb balanced at 2500 and 3500. I have flown with the balancer and there is no relationship to the ips reading and the rumble. Nothing seems to cure the "rumble". The motor produces good power and economy, plugs look good. The plane has a Dynon on the left and steam gauges for airspeed and oil pressure on the right. 

Today I had a mechanic ride with me for his opinion. His first observation was the steam oil pressure gauge was fluctuating about 10 psi. I don't pay much attention to that gauge because of its location, but assumed it may be vibration moving the needle. The Dynon pressure was moving 2-3 psi. I looked back on my logs for 4 years ago and compared to now I don't see anything different in the oil pressure but I wonder if the read rate on the Dynon may buffer a fluctuating pressure.

The mechanic admitted he doesn't know much about Rotax but said if it was a Lycoming he would consider a spun main bearing.

I change oil at 50 hours, always sport plus 4, magnetic plug has never had metal, oil filters always cut open never found debri.

I am open to all ideas.