Something making noise.

[1976Scottsdale]

I got my truck (79 chevy k10) all done and took it out o drive it and break in the engine.  When I got to work, I put the th350 in park and when I let off the brake, the trucked slammed forward about three feet, it made a loud popping sound while doing this, I was amazed that the engine stayed running.  Why would t just try to slip forward?  Also, I later took the truck out on an abandoned strip of road and had it up to about 85mph, not bad for only turning 4 grand.  Anyway, it came time to slow down to avoid traveling onto the highway, when I got slowed down to 25 or so, the trans made a grinding noise all the way to a stop, yet the fluid is fine and full, no turtle shells in the pan at all, just the usual grey stuff, and not much of that.  Now, I know that something is odd because, 1, it is an automatic, and 2, I am already aware that the modulator valve is almost shot and needs replaced.  I have to have all of the bugs sorted out before it gets much colder, so any help is appreciated.

Thanks,
Scottsdale

[1Ton]

If you haven't figured it out yet. I have a suggestion. It sounds like the linkage is not adjusted right. Make sure the gear shifter is going all the way up into park. Also check the transfercase linkage. I had a 84 that like to pop out of gear when getting on it. Body would flex and move the transfercase lever. Some things to try out.

[1976Scottsdale]

I added a body lift and had to lengthen the linkage rod, so I will have to adjust it a little more I guess.

[1976Scottsdale]

I also have my 1980 Big 10 donor truck with a th350.  As it turns out, my valve body in my 79 is kind of worn out.  So, would the valve body from my 1980 th350 fit into my 1979 th350?  I am good at engines, but I am new to transmissions and thought this would be a good place to ask questions.  Also thinking of installing a shift kit while I am tearing into the valve body.  Most importantly, how do you swap a valve body?  I hope not to have to switch this out my usual way(tear into it and try to remember how to put it back together)

[TRUCK6DRIVER]

Yes the valve body will work. But I do not think that is you problem. If the linkage is correct then it sounds like you have internal damage. My 81' made similar noises before I lost reverse. The trans would move foward fine but engine braking was gone too. If you pulled it down into 1st  when slowing down it was like neutral. What was found was the reverse/2 gear hub had spun off the splines in the clutch pack. So be prepared for that possibility.
Ray