I have just had (2) Rheem RGTM06 furnaces installed. One in 1st floor closet and one in 2nd floor attic. BOTH SHAKE THE FLOOR. My questions are as follows:

1. Is this a common problem?
2. This was a $$$ installation including new rigid ductwork on downstairs system and utilizing existing flex duct on 2nd floor and I am holding $500 right now. How do I approach the contractor diplomatically b/c the repair to this problem will probably be difficult/expensive? Keep in mind that the contractor has already explained how "he lost money on this job b/c it was more difficult than anticipated".

Pricing isn't allowed