Hmmm.....well, I still feel strongly about the looseness. The question is where and why.
Its difficult to tell you exactly where the looseness is located. Did you open the fan housing and look for any type of looseness of the fan on the shaft? Are there any signs of cracks in the weldments? With power secured, does the shaft turn smoothly with no rubbing or abnormal sounds?
Also, while its running, look closely at all "interfaces" that is, housing support to base, base to foundation, etc. Just checking bolt tightness is not always enough. In cases like this, I use my fingers to sense around for the highest impacting.
Don't give up. Jim P