My car has started making a rattling noise at high boost. Starts at about .7 to 1.0 bar, sounds a bit like a piece of fabric flapping in the breeze, almost metallic but not quite. With increased boost it gets louder but frequency does not change. Does not change with revs, nor throttle, just boost level.
Boost is behaving normally and does not fluctuate when 'rattling' starts. Had a look around and cannot spot anything.
It is a 99 UK with decat, VF24 turbo and manual boost control set to 1.3 Bar.

Any ideas?

 

It is quite a loud rattle and definitely reduces boost level a couple of psi. Sounds like the waste gate but I cannot imagine why it should rattle, the actuator is firm and the actuating rod works normally - I can hear the valve opening and shutting when moving the rod.

 

heatshield ?


common on scoobs, check the heatshields on the headers

they crack and the bolts holding them tend to let go due to corrosion and they will start to give this metallic rattle noise

 

Thanks Reggie, but it got much worse today and it is now very apparent that it is a blowing exhaust. Either RH cylinder to manifold or manifold to up-pipe. Will look later when it has cooled.

It was the manifold to head nuts. On the RH side 2 nuts were off and the 3rd halfway off. On the LH side one was off, one loose and on tight.

I guess that in the early stages the metallic rattling noise was the manifold lifting slightly under high exhaust pressure and rattling against the head and/or studs.