Car appreciation... the aesthetics, the engineering, etc

So the noise in my car...

• The sound is coming form the rear driver's side (if you drive over the passenger side there's no noise).
  
• The sound is like the noise made when you drive over a manhole cover - sort of heavy metallic du-dum.
  
• the sound is made when going over bumps.
  
  

The garage had a good look at the car - taking the underside protective covers off and giving the wheels a good shake. They couldn't spot anything visibly. My OH said that they gave it a pretty thorough looking over.

We took everything out of the boot (except the spare tyre). I had a good look at the spare and found some loose bolts that I bagged up.

If anyone has any ideas I'd appreciate it.