Motor mounts will definitely make noise, but not like you are describing. Mike has probably got it right and the only reason I mention it is you aren't listening to him and the problem can not be determined visually.

I can guarantee that if the boot is broken on the caster joint (as I call it - Mike calls it a guide tube mount) then it is bad. here is a picture (sorry about the size) that I pasted from this article:

http://www.continentalimports.com/ser_ic4232.html

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__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician