Rattle Under 1k RPM's, Only While in Drive

[fast 3.4]

So, after my header install, there has been the usual vibration that we all know and live with, but when the car starts heating up, there is a rattle from under the passenger side of the engine. I've read several "rattling, vibrating" threads, but none were quite like this. The rattle always stops when I go beyond 1,000 RPM's, and only happens in Drive. Sometimes you hear it a little bit in nuetral when sitting in traffic, but it is usually quieter or won't happen as long. It kind of sounds like a jackhammer or a jig-saw. When the engine is real hot, it gets louder, especially when I try to parallell park after a long drive.

I've been running hot lately for unknown reasons, but I have a few ideas. a flush and 160* thermo should help, but I doubt the rattling will.

From what I've read, and what I know, I have the following ideas: Transmission cooling line vibrating against headers, motor mount bolt loose, heat sheild loose, alternator shaking (speed shop didn't re-install one of the rear braces that mount to the rest of the engine for reasons unknown). Keep in mind that because this only happens in drive, I cannot go inspect it while it is happening, unless I want to look under the hood while the car is moving , and everytime I try to get someone at my house to help me put the brake on while it is in drive so I can inspect, but it never gets hot enough or no one is around.

Since it only happens in drive, does that single it out as a transmission issue?

[Camaro Dom]

damn dude, thats a hard one, but i wouldnt necessarily call it a tranny issue, cuz when the car is in drive and stopped it idles lower than if in neutral and lower idle means its gonna shake a lil more which could be causing that noise. if ya wanna get a mechanic's stethescope that might help, ya could figure out exactly where the sound is coming from. put sum bricks in front of the wheels or sumthing to stop it from moving so you can listen to it