The problem as iv found out and cured it, was the collector area of the manifolds. Basically, the collectors inlets are 4 stumps of pipe which the outer pipe of the manifolds slot over and get welded, but the inner lining slots into the collector without welding as an interference fit. The inner pipe where slotted into the collector loosens over time and has enough movement to create the rattling.
Many have had the inner pipes welded within the collector box (cat section removed for access), but a simple heat up and center punch to each sorts it (its what maserati did during manufacture of them , have a look). This tightens the whole inner/outer juction and its fixed in 15 mins, no stripping down or welding ! Someone i know did this 5 years ago and after 50,000 miles further, has no issues.
I will post a pic up of what iv done if anyone requires