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