1993 Pontiac Bonneville transmission noise

Hey some of you may remember me from a post a while back. Since then I've gotten the engine repaired and on the road but now a new issue has come to light. I'd appreciate any help because I'm stumped. Here's the specs:

1993 Pontiac Bonneville SSEI
3.8l Supercharged with tons of mods
4T60-E transmission rebuilt 600miles ago

If I'm at a stop and launch the car or floor it anywhere below about 45-50mph I get a distinct rattle sound. It sounds very much like rod knock but in neutral or park I can not duplicate the noise, even at redline. I know thats bad but I had to know.

I checked all my spark plugs for signs of detonation and there are none on 600mile old plugs. They look perfectly worn with no imperfections and perfectly white.

Above 50mph I can floor it and I won't hear a peep no matter what the RPM. The rattle does not coencide with a drop in performance either. I'm not sure what to think of this.

The noise also seems louder on the drivers side, where my FWD transmission bolts to the engine. With the speed that it goes away I would think TCC because thats where it comes into play but I'm not sure where to go with it.

The other thing that catches me is if I let off the gas at about 30-40mph the rpm's will stay at about 1400rpm as it slows and drop to 1300 then quickly rise back up. This happens in less than a second and will repeat until I get under 30mph. It will also go away if I put it in neutral. It sounds as if its studdering a little. Any speed above or below that range and it doesn't happen.

Another reason to suspect the transmission is if at about 30mph I half-throttle it the rpms will go to 2500 and I can watch a 30mph increase in speed without coenciding increase in rpms. This is what really makes me think transmission.

Things I've checked so far

Plugs - Good with very slight carbon around ring - not running lean
TPS - Off a little but fixed
MAF - Cleaned
Wires - Within resistance specifications (900miles old)
Oil - Changed 200miles ago - 10w30
Transmission fluid - Filled to proper capacity
Heat related issues - Rattles no matter what the engine temp is
Fuel pump - Pressure is a little low (40psi, 47standard)
Fuel Injectors - Cleaned once, checked again recently - Perfect
FPR - Performs properly
General performance - Runs good, accelerates strong, very slight misfire at idle.

Again, any help is appreciated. Hello DrJay2003!

Before you pull the tranny, take her to the muffler shop and get them to check the catalytic converter...just a thought.
In th Lord, Tommy