I have a Toro Recycler mower that's 2 years old. The oil was recently changed and the blade sharpened and balanced. The mower vibrates when the engine is running and stops vibrating when the blade is engaged.
Is this normal or is there another cause?
I would bet IL is correct, but would also check to see if the blade was properly balanced and that the BBC was installed correctly. It should not have been touched for, blade maintenance, but some people go farther than they should.