I have this noise/rattle I can't find.

I'm beginning to fear it's piston slap. Can some one give me a good way, without pulling the motor, to diagnose this?

It kind of sounds like detonation but it isn't, I hardly ever hear it when the motor is under a load.

If I put the trans in 3rd and hold the brakes hard to really load the engine (manual VB) it doesn't make the sound.

You'll hear it while cruising easy, sometimes on decel and occasionally at idle.

Another way to describe the sound is it sounds like a bolt rattling around in the exhaust pipe or like a loose bolt where the collector adapters bolt to the headers (would be nice but that's not it either).