Several years ago I was flying as a hired gun for an outfit that found itself tight on airplanes during the busy season. As such, myself and another pilot who based about 50 miles away were sharing an airplane. His airplanes was down for extended MX, and being the low man on the totem pole, we would be sharing the one I was assigned to. The deal was, one of us would start the nights works and work until we were done then fly out to the others strip, were the next pilot would hot seat the ship and fly out the night. The next night he'd be first one to fly. We had a pickup truck worked in the rotation so that the pilot who flew first had a way back home.
The pilot I was teamed with was a 30+ yr. veteran pilot who cranked out the work... or pushed the airplanes depending on your perspective

He was a 'pilot's' pilot kind of a guy, who knew how to handle an airplane and a race car. And could work on either equally well. A ball cap and earplug kind of spray pilot who knew the airplanes better than most mechanics, getting in one behind him was never a remote concern. Specially for a much younger, less experienced fool like myself. In fact getting in an airplane behind him (never mind that it was my airplane) was almost like having Kirby Chambliss land next to yo and asking if you wanted to take his Edge 540 for a whirl
The one fly in the ointment... Early on he had let on that he hated flying the airplane I was assigned to, and watching him near it, you could almost feel the resentment in the air as he set it up.
One night he pulled on to my loading pad just a few minutes later than he had said he would on the radio, and on climbing out of the Thrush I noticed he was white as a ghost. I asked if he was feeling ok, and he said he physically he felt great, but he just got spooked pretty bad. You'd think that'd be fairly common spraying at night, but not for a pilot who had been doing it for longer than the company owner had been alive
After a little prodding he finally fessed up that he had run up to a wire he was jumping a little tighter than he had been, and consequently pushed over pretty aggressively on the far side. This is pretty poor form, but sometimes shiv happens...At the top of the push he described a super loud metallic bang that he said he could actually feel. Not like a loose wire or tin slapping a frame tube, or a loose tie down in the baggage, this was a substantial clunk of metal. His immediate thought was that a tube had sheared in the fuselage. But after babying it through the rest of the load he could find no abnormal flight attributes, and after returning for his next load neither he nor his loader could find anything out of place with it.
Although I really looked up to this pilot's skill and mechanical aptitude, the color of his skin at that moment wasn't jiving with the way he was trying to convince me that the ship was good to go... So as soon as the little pick up pulled off the pad I set the fire to the off position, and proceeded to spend the next hour trying to convince myself all the big pieces of the ship were still in their correct locations.
Flying out the rest of the night, I treated that bird with the most gingerly touch I think I have ever handed a work plane, and I am certain I felt and heard more creepy crawlies than a moonless night could produce. At some point I thought that I could actually feel the individual strands of cable on the controls as they ran across the pulleys

. But of course, the problem never returned, and the old girl performed flawlessly.
After a (mentally) long night, I did a very thorough post flight and still couldn't find anything, so I pulled the tie downs and gust locks out of the baggage compartment. In a typical Thrush the baggage is directly behind the pilots upper body and shares a bulkhead with the seat back. The top is open to the canopy, which goes up to a small peak formed by the roll bar and bulkhead.
Shining around my flashlight around the now empty baggage still grasping for clues, it hit me like a sack of potatoes.... there still stuffed in the peak of the canopy was a quart (turbine oil still comes in old school metal cans) of oil. Literally wedged / dented into the peak.
Tremendously relieved I peed.... I resented the fact that we were short on planes, and high on pressure.... resented the fact that I got in the airplane without finding the can, and resented the fact that all of us knew the pilot well enough to know that if he felt something, he felt it... Yet we (I) still pressed on

After swearing at myself a while, I swore that if I ever found myself to be in the position of a senior pilot, mentor, or operator position, I'd do my damnedest to not let a junior man feel pressured into putting on an uncomfrotable airplane. Realistic? who knows? I guess we'll see...
Sorry for the longwinded, less than helpful post. And thank you for the reminder. I hope your clunk turns out to be equally benign.
Take care, Rob