rumble from F Expo

I have an '05 Expo with 24 M miles on her. Up until recently, she's run smoothly. Now, I'm getting a "rumble" feeling through the steering wheel, at
idle. I tried putting some fuel injector cleaner in.. to no avail. Just yesterday, I noticed that the "rumble" goes away when I shift into neutral.
I previously had a Tahoe and it had similar symptoms. Turned out to be an issue with the exhaust pipe. They had to put a "dampener" on it..which was a heavy weight, strapped onto the pipe. I guess without the"dampening", the pipe would contact the frame somewhere.
Anyone know anything about this problem? I don't want to waste a day or more at a dealer if I don't have to. I know it is still under warranty. ( I wasted three days at the Tahoe place, before I took it to ANOTHER dealer..who found it in ten minutes! ).
Thanks RG
RichG manager, Carolina Skiff Owners Group on MSN
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