Trasmission problems maybe?

[Ghettorex]

I went out and test drove a 240sx earlier today. When you put the car in first gear and take off and it his 4k RPMs, there is a grinding noise coming from the engine bay. You can only hear it while inside the car. What would be the cause of that? My friend suggested a belt but belts normally make a little bit of noise. Next, when I went to put he car in reverse, it wouldnt go in all the way. The gear would vibrate and grind a little. I'd have to then push the gear really hard into reverse. When I'd start going, it'd make a little bit of noise. I'm almost sure that is transmission problems. How much would it cost to replace the gear or would you have to do the whole transmission? If so, how much would that cost? Thanks