Renkus.
Balancing the wheels does not always show a sideways movement (1) such as an "Egg" in the tyre. Give them a good look over, preferably take them off and check.
Rear wheels can also get out of balance, and they may need doing.
Prop shaft can also get out of balance, but rather unlikely. And it would be a bit noisy too if it was that.
(1) Although a Tyre man worth his salt would have spotted it when carying out the balance.
(1a) Unless he put the guard on the machine so the tyre was hidden during it's test-spin. But again, if he was worth his salt, he should have done a final visual check without the guard on.
I don't think it is more serious than tyres or wheels. Buckled wheel maybe...kerbed it lately?
HTH.
regards