A really odd knock HELP

[93formula]

I have a 91 Talon with a built motor that has about 6000 mi on it, and when I was driving around today I started to hear a knock. So, I pulled into a parking lot and while it was idleing I could hear a distinct knock that sounds like a rod, when the motor was reved the knock quieted but didn't completely go away. So pissed off I left the car for about 3 hours. I brought back a stethoscope to listen to the knock and see exactly where it was coming from. I started it and nothing no knock at all! I drove for about 30 minutes and still nothing. Any ideas? I'm lost the only thing I could think of would be a little piston slap, but i didn't think that that happened when the engine was warm. Correct me if I'm wrong.

 

[93formula]

Update, I can get the car to do the knock, but it only happens when the cars is warm and has been driving for quite a while. Still when revved the knock seems to go away. Also I hooked up a laptop to my AEM and found that when the knock is happening it trips the knock sensor at idle but not any where else. Does anybody have any ideas?