toro proline vibration

I have a toro 21 inch BBC commercial 22043 when it sits with the blade off it seems to have this vibration. kind of whaaa sound and the blade can bee seen vibrating. But when i engage the blade it runs strong as new. Then i put all the new parts on my other 21 inch mower aluminum deck 20684 i didn't have any vibration at all . the suzuki engine just had brand new bearing installed. I am at a lose is it just having a steel deck as the aluminum?

Thanks for the help