Many years ago, I was sent to fetch one of our aircraft that had had some protracted servicing. It looked normal on the ground, started normally, and behaved well enough in departure until I transitioned from climb to a somewhat faster cruise speed.
Without any lead-in or warning, the entire airframe suddenly seemed to be involved in an intense buffeting vibration. After some number of long seconds, the vibration subsided, without my having taken any specific control action to either cause it initially or to abate the vibration subsequently.
I experimented cautiously, using power for control. Increased speed did not seem to matter, but decreasing to the low end of cruise caused a very aggressive flutter in the wings that seemed possibly strong enough to disassemble the aircraft while in flight. The effect was quite violent and noisy - much different from stall-type buffeting.
So, the good news was that I had some control over the problem. The bad news was that I could not slow down without getting the shakes, and had no info about how the rules would change with shifting balance due to fuel burn. A swift return to point of departure seemed appropriate.
I concluded the only way to avoid these shakes while uncomfortably close to the ground during landing was to make a very 'hot' wheel landing on, alas, a nearly windless day. Very long runway helped, tyres cooperated, all went fine.
The shop fussed over it for a few days, then eventually confessed that an un-named mechanic had parked his lunch pail on a pile of shims which were meant to be used when re-attaching the wings after servicing. Without those shims, the wings had enough play, under normal air loads, to wriggle around on their own, making the entire aircraft a sort of harmonica at selected speeds.