What frequency vibration?...low <1000 rpm (like a mis-firing cylinder)...medium 1000-2500 rpm (prop speed)...high >3000 rpm (a buzz).
Where does the pilot feel it...in his seat?...controls?...feet on the floor?...airframe post?
Retractable gear or fixed?
In the mean time, if nothing has been changed or modified recently...and you think it is in engine area....
-engine mount rubbers separated...hardware loose or broken
-exhaust system cracked
-prop backplate cracked...spinner mount cracked
-depending type of prop...cones, seats, seals loose