1973 F100 Ford truck tranny problem

I have a 1973 F100 Ford truck. I'm kinda new to automotive repair and bought the truck so I could rebuild the motor as kind of a learning project. When I first bought it, the truck ran fine with minimal noises. About a year later, the truck started making a noise in fourth gear (its a 4-speed 4x4 with low 1st). The noise has since then been getting worse. When the engine has a load (being accelerated), the noise is there. When the truck decelerates, the noise is there but its a little different and sounds worse. Right in between those areas, the noise goes away. Also, pressing in the clutch pedal makes the noise go away completely. The best I can describe the awful noise is like a high pitch grinding noise. Someone said it sounded like a pinon gear in the rear end getting loose, but I'm very sure it's in the transmission.

What I need to know... is this problem fixable? If not, how do I determine what kind of transmission it has and which one is best to replace it with. I'm not too worried about the price and want a sturdy transmission. Thanks for your help. It could be that you have bad springs in the clutch or a bad throw out bearing if pressing on the clutch stops it. I had a 87 S-10 (blew it up, wasn't legal though anyway so it wasn't much of a loss

) that had a horrible shaking and noise in fourth gear. It turns out that the clutch was almost totally blown out. It finally did one evening in the mud and muck. I was stuck so bad that i broke the 4x4 rocking it back and forth, about the time the clutch when was the same time that the ole` engine started knocking real bad and the next thing i knew i was walking home! I had thought about the clutch being the problem, but I was confused why it was only having a problem in 4th gear. 3rd gear has always had a howling noise (a much different noise from the 4th gear) but the person I bought the truck from said it had always done that and he had it for 10 years. The noise in 3rd gear hasn't been getting worse. The clutch probably needs replaced anyway; I guess I should look at that before replacing the transmission. Thanks for your help!