On & off slight vibration at cruise
 

I just reinstalled the engine a few weeks ago after doing some upgrades. Heads, cam, intake etc. All the Marine accessories were off, and reinstalled. At about 3000 RPM a slight vibration is felt in the boat and in the steering wheel that comes and goes in almost a rhythm. It will vibrate maybe 5 seconds, then off 5 seconds. When I feel the vibration the exhaust note also changes slightly, although there is no noticable change in RPM or speed. Any ideas?

Single or twin?
If you have twin engines, it's could be a synchronization problem.
If it's a single, check the drive alignment, might be off. Re-tighten all the engine mounting bolts, they could've loosened with the vibration. Drive attachment bolts too.

It is a single. Would the coupler show those symtoms?

an engine will have bad harmincs at certain rpm's, it being in a rhythm does seem weird. meybe something loose in the drive?

White Knuckle, did you ever find the fix for your problem? What was it?