Tranny Noise Question

[Spline64]

Hello,

Hoping somebody can point me in the right direction here.

I have a TH400 tranny in my 1973 GMC C2500. Just recently I have noticed a noise that is coming from it that I can barely hear but is starting to get louder over the last few weeks. I noticed it first when decelerating below 30 mph but now I can barely hear it when accelerating as well. It sounds like a squeaking or somebody quietly ringing a very small bell under the truck. If I place it in neutral while this is happening and coast it goes away immediately and comes back when I put it back in drive. Braking has no effect on the sound.

Ideas?

Thanks,
Pat