I'm no mechanic or even all that knowledgable about cars, however I've asked a lot of questions about knocking problems as I've had several of them myself.
If it's an engine noise, then the knocking is nothing to be worried about. I've been told it's normal for an older car and peculiar to the eseries engines.
If you think it's coming from the wheel area, however, it might be something more serious. I recently had a very loud knocking sound come from the rear right wheel whenever I went over a bump. I was told by several people that it was just the shock absorber that was dead and needed replacing. However when I went to the mechanic, it was found that a bolt was missing from... err... I *think* it was called the 'control arm'. Anyway, it was apparently quite dangerous to be driving without it.