Ebrake is clunking

About six weeks ago the brake warning light came on. I checked the e-brake to make sure it was off several times pushing down hard to be sure it was all the way down. It was. Checked the brake pads they are about 60%. Checked brake fluid it is at the appropriate level. Went about my business not thinking much of it. Last weekend I decided to test the e-brake while I was coasting. It did not do much, but after a hard pull it eventually slowed the car a bit. I released it. Then, about thirty seconds later at about thirty mph cruising speed on a flat surface, a loud clunking noise from the back of the car started. It was rhythmic clunk, clunk, clunk, clunk seeming to coincide with the rotation of the wheels. I pulled over and parked. Got out looked things over. Got back into traffic, noise was gone. About sixty seconds later, after reaching thirty or thirty five mph, the sound started again. Repeated the process. I turned around and drove home. Each time, after pulling over, the clunking went away. Then it would start back up again after reaching about thirty mph more or less. This morning I took both rear wheels off, checked to be sure there are no noticeably loose parts. There are no loose parts that I could tell. Caliper bolts are tight. The wheels were on tight before I dismounted them. Nothing jiggles. On the rear passenger side, there is a metal on metal sound when I turn the wheel hub. But I cannot turn the wheel more than maybe an eighth turn as it is locked. So, we have a brake light on, a clunking noise at thirty mph and a little metal on metal sound on the rear passenger wheel. Any ideas?