Vibration? @ 3000 rpm

Just got the 1976 kz900 running last week and am trying to get some of the little bugs worked out. I can't really tell if it is carb related but thought I would post here first.
The bike starts easy and idles low for a few minutes until warmed up, then idles strong and smooth. Acceleration is good up to 7-8000 rpm. The problem happens at about 3000 rpm. It is hard to describe but almost feels like a vibration or growl while maintaining a steady cruising speed. If I'm giving gas or letting off I don't seem to notice the problem.
At first I thought I had a wheel balancing problem but the vibration stays the same no matter what the mph speed. Any suggestions?
The bike has been sitting for 20 years and the carbs were very gummed up. I totally took them apart and soaked each one in carb dip. Set the float levels using the wet method. Synchronized the carbs using 4 vacuum guage setup. Installed fuel filters...

Does is buzz when it's on the kickstand, and you rev it to 3000?
It it does, then you may have a loose engine mount. Your cam chain might need adjusting. Do you know how?

Hmmm... Just went outside and checked, yes it does feel the same at 3000 rpm sitting on the sidestand. I'll look at the mounts and see if anything is loose or broken.
I did adjust the cam tensioner following the instructios in Clymers. I did that at the same time I checked the point gap and the timing. Also pulled the points mounting plate to check the advance springs and weights. They all looked good and the timing advanced when raising the rpm.

THANKS

Lots of things can buzz.
Clutch/Brake levers
Mirrors
License Plate Frame (that one took me a while to find, I can tell ya)
And sometimes they're pretty loud.
It's hard to isolate when the bike is moving, but easier in the driveway.

Try checking the gas cap... Sounds stupid but mine rattled terribly and the tank magnified the sound. To cure the rattle-vibration, I reversed the rubber gasket and the noise went away.
Grant.