I had an old steel wheel on a 70s UO-8. The bike had been left outside in Galveston Texas air and almost all of it was well rested. The owners husband had slobbered the entire rear wheel and freewheel with Marine Grease so it actually looked pretty good after I got all the dirt off of it. I too had a strange sound coming from the wheel when under strain but it was a long wait before I could afford a wheel set so I babied it along. Oddly one morning, as I got off to class, I noted three spokes popped on the rear wheel. When I took the wheel off to repair I noted it was headed for a catastrophic failure. The rim was literally coming apart. The inside of the rim and bead were rusted through. Also the weld holding the wheel to round was cracked. It was a nice looking wheel but totally toast on the inside of the wheel lip.

It looks like you have isolated the problem to the wheel. There could be small cracks in the alloy or spoke nipples in loose fittings. It could be the wheel was just made of a bad batch of alloy or the temperatures were not right at manufacturing.

Don't wait for a catastrophic failure. There could be a defect in that wheel you won't see till its too late...