So I have a 2001 SR5, 4X4, Auto, 275K. Transmission was rebuilt in July and I have since driven about 9K miles on it.

The sound is like an electric motor whine, coming from vicinity of transmission. The sound is not audible or present when in Neutral or Park. I hear it when in Drive or Reverse, with neither seeming more noticeably louder. It seems that it may increase with RPMs, but when I accelerate the whine is muted by the overwhelming roar of that 3.4. No slipping in transmission and shifts seem normal. The truck is in ECT Power mode.

I have a power steering drip I manage. I have listened to the ps pump and it does not sound like it is the culprit.

It also got real cold here in Denver a week or so ago, and there was a small amount of coolant that leaked out when it was single digits. I believe the drain cock was loose. i tightened it and the radiator was full, topped off the reservoir.

Just sharing the last two bits of info in case they become relevant.

Is this RC sound from the transmission? If yes, what could it indicate? If no, what else should I be looking at/worried about?

I'd take it back to the builder and ask them to judge, if they say it's ok and things go bad later it's on them, and you can go back and say, see told ya it sounded funny, if you drive it and don't tell them they'll be like where ya been why didn't ya bring it back, operator error,-in some cases depending on the shop. Wouldn't hurt to check fluids running at operating temp, and do a drain and fill(a lot of debris gets in there when you start working on them) inspect oil ect, any findings show to builder, confirm you have some regular old dex3 or similar, hopefully they used a OEM filter. Your description reminds me of driving in 4Lo, so maybe check all those fluids are good before visiting the tranny shop so ya don't look like a chump that forgot to check the t-case for the past 19years. Did you do the install?, if so all alignment pins in place, everything tourqued to spec

The shop that rebuilt did the install. I will make sure it is nothing obvious before I drop by the shop for their take.

I made sure there was nothing obvious causing the noise. Then I let the truck sit for a couple weeks. This morning I brought it in and the shop diagnosed, from the sound and a test drive, that it is coming from the torque converter and is likely a bearing. They are warrantying their work.

I will follow up post repair.

Thanks for the information.

Good luck with the repair and thanks for sharing. I wonder - has anyone here ever successfully replaced JUST the torque converter?

They are simple to remove from the transmission, however the transmission itself it not simple to remove. Once you unbolt the torque converter from the flex plate on the back of the engine, then unbolt the transmission housing from the back of the engine, there's no bolts holding the torque converter in the transmission, it just slips into place. I wish there was a way to remove it without pulling the entire transmission but there is not so it's the same amount of work to replace the transmission as it is just the torque converter.

You can, but generally speaking when a torque converter fails it usually causes damage elsewhere in the transmission unless it is replaced quickly. Either the fluid gets burnt and no longer lubricates properly or it adds metal to the fluid.