I'd lean toward prop issue rather than "engine" issue. Best move, find someone with balance gear if possible and do a dynamic check on prop and a spectrum check on engine. If it really is a engine problem, i.e. no accessory mounting issues, prop, etc... then don't fly no more. Any true engine vib problem is normally crankshaft issue...weight slung off or worse a crack. Does it have a VAR crank?
In the meantime....
When g load applied...does vib get worse or better with increase in throttle?
What kind of prop and how many blades?
On ground...static (not running)...check prop blade track. Put reference item...tool box, chair, whatever next to one blade. Mark lead or trail edge position on ref item...then rotate to next blade and see if track is close...repeat with 3rd blade if necessary. For more info:
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/list/AC%2020-37E/$FILE/AC%2020-37e.pdf
Exhaust, prop backplate, spinner, alternator, etc all serviceable?...no mount cracks?
Torque on prop bolts within limits?
w1