Thanks Tomp,
Much appreciated. I've taken the header off about 4 times now. The first couple of times I thought it was the flange gasket. I used high temp silicone for exhaust to put around the gasket but to no avail.
The last time when I took it apart, I noticed a small dark spot seeping out of the collector/exhaust pipe connection. I hadn't used any gasket silicone because it uses the GM o-ring there.
Needless to say, it didn't help.
Yes, I understand how headers tend to allow the valves and injector's noise to sound like it's got a leak. Mine is definitely a small leak and is proven when I step on the gas. Idling is hardly noticeable, stepping on the gas sounds like a singer sewing machine.
One last question if you don't mind. When you say it had a leak on the collector... do you mean where the collector flange was welded to the actual header pipe?
I'll try the stethoscope trick but it's hard since it really needs to be under a load to hear it.
I wonder if I can get TTK (Tim) to hand on to the undercarriage to help me find it?
Thanks for your time,
Mark