Amazon: Clicking noise from rear wheel

[HRA]

For a long time now I've noticed a clicking/tapping noise coming from the rear offside wheel. It's not unlike someone flicking a plastic cup. At first I thought it was an intermittent problem but now I've realised that I need the driver's window open to hear it and I need to be driving close to something like a wall or hedge to get the noise echoed back to me. I get the impression that it is louder than before, but maybe I'm just more sensitive to it.

The clicking is rhythmic and varies with the speed of the car. I get the click in all forward gears, coasting in neutral or coasting with my foot on the clutch. At a guess I'd say it was one click per turn of the wheel, but it is only a guess. Interestingly, I think I only get the click when I drive forwards - I can't hear any clicking in reverse.

I can't see anything fouling the wheel while it is on the ground. I've jacked the car up and turned the wheel both ways by hand but there's no clicking. I've jacked the wheel off the ground, supported it under the axle, started the engine and run the wheel in first, second and reverse gears but still no clicking. I've repeated the exercise with both rear wheels off the ground - no clicking. But as soon as I've got all four wheels on the ground and I'm driving there's the click again.

I've had the rear brakes overhauled at the local garage but the clicking is still there. At the same time I put the rear drums up on a lathe and turned them "true". (There was plenty of metal there so I'm confident I haven't made them too thin.)

I've topped-up the gearbox oil and the differential but that's made no difference. The wheel bearings feel OK (no obvious play in the wheel) and there's no growling noise of dying bearing. The noise has also survived a new tyre, and I think it was still there while I was driving around on the spare wheel.

I'm at the point of giving-up and taking the car back to the garage, but I imagine that searching for such an elusive problem won't be cheap. Before I do that is there anything obvious I've missed? Has anyone had (and cured?) the same problem?