Pinking - How can I stop it?

I've been thinking, could it be the cat heat shield? It makes a rattley noise when the revs (40mph in 4th usually does it) are right but could it be making a different rattle when under load? You will probably need to have owned a Sei/Cinq to know what I'm on about but any opinions would be appreciated.

You can try a bit of loft insulation to damp the noise & find out

Yes it could be, but it's a _completely_ different noise to pinking. Melissa did this, lots of cars can do it actually.

Get another opinion?

I'm right! It is the cat heat shield. I had a listen to the noise and sometimes you can hear that it's a really sudden and quick rattle. There is also a longer and really long rattle between the loud ones. At the right revs it sounds completely different but at low/mid revs it's hard to tell. The 'pinking rattle' only happens under load though.