ok so i purchased this used recorder the boss br864 a pretty good deal but the thing is when i turn off the noise gate it buzzez ALOT when i turn it up the master fader up past 1/3 the max and when the gate is on and i play you can hear the buzzing in the background anyone know a way to fix this or should i just bring it back?

Te previous owner probably sold it for a reason... go back to the shop and show it to them.