I have an 86 300/C6. I pulled the C6 out of an 81 and had it totally rebuilt by a local shop who was a friend of a friend. The tranny had gotten wet and there was many hard parts that had to be replaced as well. I think it cost me nearly 700 several years ago.

The tranny seems to work fine. I towed my race car 100 miles one way to the track several times with no problem. I have a huge tranny cooler BTW. It is at least 1ftx1ft. I forgot the gross vehicle weight of the unit.

Issue is a noise that comes from the tranny. When in neutral it whines like a low power steering pump. As soon as it engages drive, the noise quits. Reverse gear sounds like a manual doing 20mph backwards.

Its been like that since rebuilt. Guy who rebuilt it claimed that I damaged the pump when I installed it. I know that is possible, but I have done this before.

The truck was factory 3 on the tree. I swapped in the c6 because the 3 speed couldnt handle 31 inch tires, 3.08's and the load of the car. Didnt make it 2 miles and totally smoked the clutch.

Any thoughts of what the noise could be? I am nearly ready to sell the truck, but I fear the noise would ward away any buyers.

 

It would seem to me that if you damaged the pump on install at least the front seal would be out and leaking as well.
Could be low on fluid, wrong type of fluid or perhaps the torque converter is going. Get a book on it and do some of the pressure checks, that might lead you to the right spot. Good luck.

 

If you damaged the pump on installation it would not have lasted this long.

You could have valve body concerns. A blown gasket for one.
Is it full of fluid?

 

That's what I mentioned before. I put in three all in one day in the same truck once and I've bumped the input damaging the front seal. Unless you could somehow get it started in wrong and tighten some bolts, I don't see how you could have damaged the pump.
Concerning the valve body, I learned the hard way about the filter/screen. If you drop it out, take it out that is, you will find a small screen/filter on top. This doesn't come with the filter kit and unless you have a broken screen on either one, there's really no point in purchasing new ones. Just take them out and clean them with hot water. Let them dry and put em back in.
As for the fluid. One thing I do know is that the 80-86 model C-6's don't use type F. That was the cause of one of the three I changed that day I mentioned.
If it works right, changes gears, goes through them automatically as well as manually, then the valve body is most likely ok.

 

Is it a new converter? I have heard them whine like that before...