Transmission question(s)

Hey guys

I was up in Phoenix this weekend at a swap meet. On the way there and back I noticed that my 5th gear is fairly loud. What I mean is there is a like a "roar" almost but only in 5th gear. If I down shift to 4th and hit the same speed it does not make same noise. It also has a very loud vibration in the stick that can be heard.

Secondly on the way back while passing someone on the freeway I reached about 105MPH, when I released the throttle the car popped out of fifth. It went back in with no trouble but it freaked me out a bit.

So I'm wondering has anyone ever had these issue or no anything about them. I'm going to check the fluid level tomorrow but I wouldn't see it being low. The transmission is a 5 speed out of 78 280z (supposedly) but the car originally had 4 speed.

Thanks

Jan