Im tackling my winter projects and I have a question.  My 81 has 65k miles, the drive line is totally maintained without exception.  I have the number match L81/TH350c drive line.  Over the summer I redid the trans with a B&M shift improver kit and a 2000 rpm stall converter.  The original stall converter failed.  At that time I had the trans mounts replaced.  When the car is in reverse, I have a whistle.  It doesnt do it in any other position.  The trans is strong and shifts crisp and proper.  No seals leak at all, the fluid is fresh. Ideas?

Does your TH350C have a vacuum modulator in it? If so, maybe the or a vacuum line could be lose or lose fitting.

This is just my thought here:

By shifting in reverse the drivetrain torques the opposite way maybe causing a vacuum line to be pulled or stressed a little. Then shifting back to drive may allow the line to relax back into position.

Can you have someone assist you in searching for the whistle noise? If you can isolate the general area it's coming from that would be the first step in fixing it.

Sarge  

Take Sarge's advice first.  Then see if the noise is actually coming out of the trans it's self.   Some shift kits can cause a noise due to fluid flow through the passages.  If it has done it from day one of the repair, and there is no operation problem, you may need to redo the job, or live with it.  Contact the shift kit manufacture, and see if they have any comments.  It could save you a lot of greif.

Hey Guys, Thanks for the help..  The whistle was there before the shift kit was installed.  I'll check the above at my Bro's station, up on the lift.  KeBo..

How loud is the noise? Does it change with rpm? Is it louder inside the car or outside? It may be something with the modulator/vaccuum(all 350/400 trans' have modulators). I'd try to get someone to help locate the point of the "whistle", as Sarge says,and go from there.
Believe it or not, I have seen some PCV valves create a "whistle" noise from the rear-main seal! If the PCV is stuck open, it creates a large amount of vac. inside the engine, and will actually draw air into the engine from the rear-main seal, and the seal will whistle!!