2001 Renault Megane Rear clunking noise - Renault Megane

Hi,

I wondered if anyone could help. I have a Y reg 2001 1.6 16v Renault Megane Dynamique + that has covered 115,000 miles. So far it's been a very good little car, but recently (within the last 2 months) a knocking/clunking noise, which is slowly getting worse, from (i'm almost certain) one of the rear wheels has developed. It seems to go in motion with the wheels and the speed they are going, so the faster the car goes the quicker the knocking/clunking noise. I dont think it's the shock absorber or rear bushings as it only seems to go with the speed of the car and not the bumpiness of the road. Once I go fast enough (above 40mph) I cant hear it any more. I have had a local friend mechanic take a look and he doesnt know what it is. We also dont think it's a barring as they usually produce a loud humming noise when they wear out and there is nothing obvious under the car, i.E. Something catching. Oddly though it's a little quieter when it's wet but very loud when it's dry. We even put it on a ramp, jacked up the rear wheels and span them but we heard nothing. It really is a mystery and is something that is driving me mad!

Please has anyone got any ideas.

Thanks

Higgsy82