Knock at 6000 RPM?

[Jon Pennington]

'94 G20 5sp with 207,000 miles:

Four weeks ago, the weather changed from over 70° at night to less
than 50°.  I noticed that at nearly 6000 RPM to nearly 6200 RPM, at
WOT, there's a knock or loud rattle, even during the warmer part of
the day, over 70°.  The plugs (PF), cap, and rotor are Nissan parts,
less than 25k miles old.  I changed the plugs anyway (BK), changed the
oil, and noticed no change except that I could induce the same
knock/rattle at lower RPM points and with less throttle if I played
with it.  The PF plugs were white and crusty on the side electrode,
but the points of arc were perfectly clean.

I also have a sawing noise from the HSg5 header flex pipe rubbing on
the Carbing lower control arm brace due to my lack of a front motor
mount.  Honestly, I'm still not sure if the noise from the engine is
preignition or just a loose fastener, but it's awfully loud for a
loose fastener.

Last weekend, I changed the valve cover gasket(s), and also installed
a '91 intake cam.  For the first four days, ALL noises went away.  The
exhaust stank of unburned fuel, but I noticed all of the stuff that
goes along with a highport cam in a lowport motor; slightly softer at
the bottom, much more fun in the top half of the RPM band.  In the
last couple of days, though, the 6000 RPM knock/rattle has come back,
as has the sawing noise from the header.

Assuming there are no codes stored in the ECU, what could be wrong?
Wouldn't the ECU tell me that the EGR valve was stuck open?  I just
cleaned that damned thing last September; can it be that dirty again?
Can I plug a port somewhere or pull a vacuum line to test my EGR
theory?  What did the ECU un-learn from the old cam and re-learn with
the new cam?

-- 
-=|JP|=-     <//><

Why, yes, I *can* cross-thread a ketchup bottle.