This is a bit of a weird one and a bit difficult to describe but I thought I'd give it a go anyway. I'm having a rhythmic creak/click type noise from the rear of my old 70s road bike (I know it's the rear as if I change gear at the front, the timing doesn't change). It happens most when I'm in middle gear (of 5) but also to some extent in the adjacent ones, and mostly when I'm going up a gentle gradient. If I start out, it may not happen for half an hour, but then suddenly start (maybe because I've hit a gentle gradient). When it's happening, I can ease it by releasing pressure on the pedals for an instant, but it usually soon returns. The rear cassette and chain were replaced recently. The only thing that occurs to me is the bearings on the rear wheel hub. I replaced the ones on the drive side and I noticed it was very difficult to get the right amount of tightness without causing friction or looseness. So my next step was to replace the rear wheel, but I thought I'd see if anyone can suggest anything else first.

thanks