Possibly a better vibration dampening option

This is probably a better option than the "great stuff" spray-on foam.  I read a little bit of the MSDS for the spray foam and it says not to use it with metals.

http://www.silentrunning.us/html/products.html

Expanding foam for use in vibration dampening

Just wanted to post this link for some expandable foam-in-a-can which we may want to consider putting inside of square aluminum tubing.

http://greatstuff.dow.com/greatstuff/cons/choosing.htm