Pinging or a tink tink tink noise, like a small hammer hitting my tranny

[Narkoleptik]

I hear it when i shift into reverse or drive, and it seems to sometimes make itself known while i'm actually driving down the road. It goes away when i let go of the gas. I remember someone asking about this long ago and I was wondering if anyone can give me some advice or point me in the right direction.

It's a 69 GMC 2wd 350 with a TH-400. Any help would be appreciate and thank you in advance.