Try tightening up the bolts that hold it in place and if the noise persists, tighten the motor mounting bolts and if that doesn't help, do yourself a favour and change the whole thing.
Courtesy of Ren Molnar, Construction Consultant at HomeFocus.com. I can also be reached at (613) 748-3003 and Home Focus phone-in is broadcast live 8 to 10 A.M. (Ottawa, Ont. time) on Saturdays on www.cfra.com.