Put a screwdriver handle to your ear and find out exactly where the noise is coming from by touching the end of the screwdriver to each end of the motor, the gearbox and anything it drives. The screwdriver will act as a kind of stethoscope so you can track it down. It could be the motors bearings, the coupling, the gears in the gearbox (noise in the middle), the gearbox bearings (at the shaft ends), or it could be something that just has some loose bolts and is vibrating.
Most low speed gearboxes will use some very thick or heavy oil, it could be as high as 120-140 viscosity. You should check with the manufacturer if the box is not labeled and never overfill it. Some times people will fill them to the top, very bad. There should be a pipe plug in the side for the level, fill from the top until it runs from the side hole, don't overdo it. Let the extra run out, then replug.