Excessive vibration

I have a Toro 20333 mower with a B&S engine. I tried to remove the plastic clutch cover to clean behind it but couldn't figure out how to get it off. Since then, the mower vibrates when the blade is NOT engaged, but runs smoothly when the blade is engaged. I'm not sure that this was the cause though.
Someone suggested a bent crank or drive shaft but I would think that would cause vibration all the time, not just when the clutch is disengaged.
Any ideas?

The clutch that is in the mower may not be working properly. It may be slipping and engaging the blades a little even when the lever is not pulled. If the crankshaft were bent, it will still cause excessive vibration even with the blades engaged. I'm not totally familiar with the newer Toro models, but I have seen this situation on other units. Hope this helps.

Let me know if you have any further questions. I will be happy to answer them!

check blade balance, possible coincidence?