Manual Transmission vibration in 5th Gear

[PhantomLord]

My W124 E220's 5 speed Manual gear box was recently overhauled, it shifts and runs pretty smooth. But after certain speed (say 60MPH+) in 5th gear I can feel vibration on the floor of the car (sound's Burrr...), it gets worse with increased speed.

Vibration goes off if I remove my foot of throttle and I don't see it if I do same/more speed in 4th gear, can anyone please suggest what I should be looking into?

Thank you