Huh?
I get a rattle (sounds like knocking - but could be a loose heat shield?) after install of the header/gutted first cat.

I know the gasket between the header and cat leaks (need to swap it).

So. - could all of this caused pre-ignition? Or is it totally unrelated?

I tried a fill up with 93 octane (full tank) and it would still 'knock/rattle'.

I am still awaiting my engine mounts - could the engine be moving the exhaust to the body?

puzzled.

Thanks!

 

maybe the header caused the exhaust to shift a bit, and when it did, the exhaust is now hitting the heatshield underneath your car. the only way to really tell where the sound is coming from is to have someone accelerate your car while you poke your head around to see if anything is moving. if your engine mounts are bad, then that might be the cause of your rattling heat shields.

 

Ahh - that makes sense!