Strange Case of the Vibrations

[Pheonix]

I'm not sure if this is the right forum for this question, but if it's not I'm sure the Mods will over it to the appropriate area

Anyway, lately the Z has developed an odd vibration that manifests itself most strongly coming off acceleration or on uneven roads. On cruise control it's barely noticeable, it's not really noticeable going over speed bumps or grates or anything likely to jolt the exhaust...which is where my initial speculation about the origin might be stemming from.

However I've checked my exhaust. There is nothing touching any part of the under carriage at all, and all bolts seem to be tight. I've had a friend, a fellow forum member take a second look and do a ride-along with me...and the only thing that we could find was the left exhaust tip was slightly loose...for the record, it's an AAM GTR Titanium tip that is only slightly bigger than my AAM 2.5 Dual Exhaust...but it fits. We tightened it again and the vibration is still there.

If anything, it seems to be getting stronger and more noticeable over time, and I'm starting to get pretty concerned.

Has anybody ever had anything like this before, or heard of it?