Hi from Doug Owen   
I had a similar irritating vibration - I got to the point I did a vibration analysis - I cant recall the name, but the analyst was out of Burbank.  Let me know & I can get the name if you wish to pursue it further.

Short of it is, my vibration was exactly 3x the rpm of the engine. 3 blade prop --> conclusion was tip vortices hitting the airframe.

I found I could smooth it out by various combinations of RPM & Power setting.

Good Luck