I have quite a severe knock on the n/s front when going over decent bumps but only when it has been raining. The car will do it immediately from standing and it has rained since parking. As soon as it dries out-silencio. The car is an 03 SDI with 136000 on the clock.

Any ideas.

PS I've had the front suspension checked by 2 indies who haven't found anything and there have been no problems with MOTs in the 3 years it has been doing this.

Strange one. I would check bottom ball joint dust seal, and the drop link seals. Or, wait till it's dry, and spray each component/area with a water bottle spray, and see which one causes the noise. One final thought, do you have a sunroof? I had knocking on a Golf GTi only when raining, and it was the sunroof drain pipes tapping on the A pillars.

Mine was the same for a couple of years, then it started to be all the time. Turned out to be the anti roll bar bushes, the sleeve had worn and slid out of the bush

I jist gave them a shot of tiger seal in the gaps, and they have been quiet for nearly a year now