My 53 reg fiesta flame has started making a really annoying rattling/knocking noise which sounds like it comes from the rear drivers side.

It doesnt do it all the time, it may start 10 miles into a journey. Sometimes if i drive somewhere, it will do it, then half an hour later it doesn't.

I put it into a garage, who couldnt find anything wrong. They said the suspension and exhaust were fine (although i suspect they just had a visual check). They said it may be body panels moving???
The car has since sailed through its MOT.

To describe the noise, it is like a rock rattling around in a metal bucket.

Any ideas? It is getting really annoying.