912 80hp vibration bzzz under 4,000 rpm

Thanks, Jacques 

I checked the rubber protection today and it was still OK.

If you have a prop guard on that trike make sure all bolts are tight and also check your exhaust, especially the springs, do you have the red silicone in them to dampen vibration. Just two more thoughts.

Problem solved.

It was the gear box.  I removed the gear box and sent it to our repair agent Wal at Bert Flood.  Wal pulled it part and found that all of the gears box parts needed replacing.  The BZZZZZZ was the slack in the gears when I reduced the rpm.

He renewed all parts and included a clutch to prevent it happening again.

Thanks for all the proceeding comments.