Carb overflow leaks

Check valve clearance (and see if the intake seals off right)
Check the condition of the distrubution chain and tension.
Check and replace if necessary the points and condensor and set these correctly.
No slip on the clutch?

Also, see if you're wheels are turning freely with all turning parts in clean, good condition.
What about the tension of the bolts holding your engine in the frame?

When I first got my bike I had a terrible noise and it thought it was engine related. When the revs got up high it felt like the bike was going to blow up I tightened up the engine mounts and swing arm and the noise vanished, it felt like a completely different bike.