I have a MK2 Octavia with a strange rattle/Rumble from somewhere on the passenger side of the car i notice it most when i have low revs and the engine is starting to struggle. it sounds like its coming from the rear passenger wheel area.

 

i have had a look if anything looks lose but couldn't see anything.

 

any ideas???   

Don't slug the engine?

 

Or if you want to know what the cause is, look very carefully any place the exhaust gets close to the body, and check the mountings while you're doing so.