1999 chevy 5.7 liter, Z-71 pickup knocking
I have a puzzle I'm hoping someone can help me with. My son was driving my 1999 Chevy Z-71 5.7 liter auto transmission. He said that it got to where the transmission would not shift, as in when in drive from 1st to 2nd. He got it home and parked it. When I got in from work, I started it up, the motor sounded good as always, but when I tried to shift into drive, I could feel it shift, but it wouldn't move and made the sound of gears slipping, put it in reverse and it seemed to back up. With no money to have the truck looked at, it sat for around 3 months before I started it. It started just fine and the motor sounded good. I didn't try the transmission. 3 months later I started it again, and it sounded like someone was inside of the motor with a hammer on the passenger side trying to beat their way out. I let it run for several minutes but it didn't get any better. Today, about 4 months from the last time I started it, it started just fine, with the same knock. I let it sit and run for about 30 minutes. The engine smoked for a few minutes then cleared up. I put the transmission in reverse and it again wouldn't try to back up and sounded like gears slipping. I placed it in 1st and it went forward. I drove it around the yard for about 5 minutes. I stopped, held my foot on the brake and gave it some gas, the knocking stopped and I didn't feel any slippage in the transmission, soon as I took the load off, the knocking started again.
The truck has around 200K miles on it. The transmission was rebuilt at 100K miles.
The questions I have are: What would make the trans not shift from 1st, 2nd, 3rd ect? What could be making the slipping noise in reverse? Is it a rod or main knocking? and why would it start knocking after setting for a long time? All help is very much appericated.