[Espeefan]

I always set my pinion mesh so that the pinion gear has just a tiny bit of backlash clearance, when I rock the spur gear back and forth. The old trick of using a piece of paper to set the mesh works out just about right. I like to set the mesh a little tighter then that. However you do it, the important thing is to make sure that the pinion gear is not bottomed out at any point, in the teeth of the spur gear. Rotate the spur gear and check the mesh in more then one place. Sometimes the spur gears aren't perfectly round, and they will have tight spots, when you thought you had the mesh perfect. Find the tightest spot, and set the mesh in that area. Then you'll be good to go. Just double check to make sure the spur gear doesn't wobble a whole bunch, as though it's been seated properly on the shaft, nice and square. Make sure the spur gear shaft isn't bent either. Unlikely, but I've seen it happen on the go-fast RCs.

What kind of 4:1 reducer do you have? I would open it up to make sure the mesh of the gears in there is good. Not much you're going to do to adjust them, as their shafts are ball raced, but you might find the gears are either dry and have no grease, or there could be bits of metal shavings floating around inside the reducer box.

About the only other thing I can think of, that would make noise like that, is maybe your pinion gear's shoulder is rubbing on the spur gear. In other words, the depth of the pinion gear is to shallow on the motor shaft.