Ooooooooooold thread, I know, I know...
For anybody that is having vibration problems with the Big Maxx heater, check out the main fan motor. Mine has been really noisy since I installed it. I have tried tightening everything, replaced the rubber fan guard washers with felt, even swapped the original blade for a heavier one I found on McMaster-Carr. I happened across a motor that looked to be similar to the fan motor and decided to try swapping it out. Unfortunately the rotation was wrong but it was dead silent without any vibration for the few minutes it was running.
The closest replacement I can find online is from Dayton and a new one will be here Saturday

Hopefully it is quieter than the POS OEM motor.