Here's my situation.
While driving to work my engine developed a loud knocking noise.
It sounds like a rod with a about a 1/4" of clearance.

When I first start the car, there is no noise for about 10-15 sec.
During that time I can rev it and it sounds fine...then the knocking.
I can do this repeatedly.
If I drive it, the noise will go away if I lug the engine going up a hill,
as it starts to gain RPM it starts to knock.
If I go down a hill and leave it in gear with no throttle it goes away.
If I give it ANY throttle it knocks. This knock is definately
RPM related.

I Have:
Drained the tank and added fresh high test fuel.
Compresion tested it and it was good.
Removed plug wires one at a time to isolate cylinder.
Installed new plugs. When I did it ran fine for about 2 min.(almost though it was fixed. Hasn't done that since. 10-15 sec.
Checked timing belt and tensioner and timing.
It doesn't seem to be down on power.
Dropped the oil and it looks fine.
My engine does not have a EGR valve.
Have driven it 30-40 mi. or so in this condition (trying to figure it out)before dropping the oil.
It appears to have good oil pressure by looking in the valve cover.
I am absolutely stumped and don't feel like pulling the oil pan until I have some idea of what might be wrong, and the fact that it goes away has me puzzled. Engine has 200000 miles on it.
Any thoughts would be greatly appreciated.