Check out the Titan II combine wiki.

http://agtalkplus.com/?q=node/1364

I have included Plowboys excellent diagnosis of the grinding noise in the wiki and pasted it here.

Excellent Diagnosis Of Grinding/ Vibration Sound Under The Seat Of Titan Combines.

Submitted By Plowboy

"The cab is hitting the lifting eye on the PTO. Lift the seat and the panel below it. Look under the console where the shifter is. Remove the lifting eye which is back there about as far as you can see/reach. Noise will cease.

The right way to fix it is to fix the cab and engine mounts"

From the original post on NAT here.
http://talk.newagtalk.com/forums/thread-view.asp?tid=125535