It was discovered that two bottom panels of the tailcone, directly behind the baggage bulkhead, were oil canning badly. I made a quick fix by riveting two angles there, look closely and you can see the additions. No more rumble! While looking around for trouble, I found it! Missed 20 rivets! It was a portion that was some pop rivets and some squeezed ones, apparently I never got around to putting the squeezed ones in! Corrected it as you can see from the photos 2 and corrected in 3. That is the hole that the control stick comes out..