Sounds like audible detonation.
Stay out of it!!!
Main bearing knock would be at a consistent time and speed up with engine RPM. You'll also have very low oil pressure.
Detonation will sound a lot like a bunch of midgets are under your hood shaking rattle cans of paint and you'll feel the power drop off drastically as the computer pulls timing out.
Get a fuel pressure gauge and check your fuel pressure at speed (try to keep it out of detonation).
If you have a ScanMaster, post some readings. if you don't have a ScanMaster, buy one. Ditto on boost gauge and reading too.
A quick thing to check is the vacum hoses passing under your turbo and connecting to the turbo and waste gate actuator. If they are rotted (like mine were) there's a big part of your problem. Check and replace all of your rubber vac hoses if you have not already.
Get a good fuel pump and fuel filter in the car too, then hotwire the fuel pump.
You could also quickly check you coil with a ohm meter. Measure across each coil....should be around 12K Ohm.
All the above contributed in gang style to my detonation issue.
