Who knows auto tranny's?

[Storm Chaser]

Well, I finally got the engine timed and running smoothly, so I decided to take it for a spin to see if the transmission that came with the truck was going to work out. After spending forever to find a crack in the vacuum line, I finally got it to shift correctly.

But here's the deal- when in is in first, you can hear a rattle that kinda sounds like a lifter rattle, only it is coming from the tranny. The rattle increases with speed, and right before it shifts into second, the rattle gets real quick. The rattle quits about a half second before you feel it go into second. After that, there is no more rattle and all seems well... until you come to a stop and the tranny has to start in first again. It happens EVERY time and only in first.

It doesn't seem to be affecting the operation of the vehicle, but I know this isn't right. I don't want to have a full load in the back and have the tranny go out in a bad part of town or something. Can anyone tell me what's going on? Is it easily fixable, or is it internal (ie junk)?