Ugly CIM sound in WCP 3-cim gearbox

[Oblarg]

So, our WCP 3-cim SS gearboxes finally came in recently, and yesterday we were doing our first drive code test on our finally assembled AM14U with 3-cim gearboxes.

Unfortunately, some issues emerged and in the course of ruling out various problems we decided to check each CIM individually, running straight from a battery. A couple of them, as it happens, are making a really ugly pulsating sound when they run. It's unnerving, and we're not sure what the cause could be. Additionally, we notice a not-too-pleasant smell upon running any of the motors (before anyone asks, our gearboxes are, indeed, greased) - though this is my first experience with open gearboxes, so a hot-metal smell might be normal, I'm not sure. Also, when running only a single motor, the peak speed reached is far under (about half, by my count) the ostensible free speed of the motor, though this might just be from the required torque to turn the other two CIMs on the gearbox.

The only thing that I know could be wrong with the gearboxes is that they did not come with wave washers, which were mentioned in the assembly instructions, but I have a hard time seeing that as the problem as I've never needed a wave washer in a gearbox before and there's not much play without it. For reference, the gearboxes are standard, non-WCD.

The CIMs are new, and the only damage I could imagine having been done to them is if our gearbox mounting screws went too far into the holes in the motor (we had to cut our own to size, since the ones that shipped with the gearbox were not long enough to pass through the extra 1/4'' of material we needed to mount the gearboxes to the AM14U, but I find this doubtful as I'm pretty sure they're going less than 1/2'' deep.

Any ideas as to what could be causing the problem?

As a side-note, I really don't like the fundamentally hard-to-service design of the kitbot this year - swapping out a motor or a belt is a major endeavor.