[COBALT]

The reason why I think it's the torque converter is that the sound is worse during warm-up when the RPMs are high and pressure in the transmission case is at it's lowest. It doesn't seem to be better or worse under load at all. It'll do it sitting still from a cold start.

I'm thinking both halves of the converter are rubbing together, which means the lock-up clutch in the center is warn out. Any debris dropped by the converter will contaminate the valve body of the transmission over time. It's seen a lot of miles even before I bought the truck in 2001. Then it was the best thing in the truck before I did all of the work to get it running. I've been suspecting it's due for a rebuild for a while now. I've been babying it for 30,000 miles, and it saw some serious miles before that, so I think it's seen it's limit.