Capacitor in parallel with motor - What happens if I remove it?

[Gyro]

Your Instek fan had a drive frequency of 65 Hz?   

Yep, a great piece of design, it interacted nice and randomly with the fan commutation as the speed varied. About as annoying a rattle as you could get! 

[WB4IVG]

I have ran into this exact problem many times. This is in fact a Hash Filter cap. The inductive kick back from the motors is likely what is damaging the Cap. In my experience What I have done is to go to a larger Voltage capacitor i.e. 16 Volt or 25 Volt when I have done that the problem went away. It also it won't hurt to go a little larger in Capacitance value. If I am remembering correctly you can go up 60% or down 20% without determental effect except in a RC time constant circuit.

[paul8f]

Just to draw some sort of line under this thread...

Turns out the 12v line feeding the motors is not constant. This voltage varies, which in turn speeds up or slows down the fans. So it looks like it's not PWM from the MCU after all (my scope was picking up interference).

There's some sort of switching circuit that can swap between a regulated 12v and the 12v direct from the inverter battery. Then, depending on the electrical load and whether or not the battery is getting charged, that voltage can vary anywhere between 10.5v and 13.8v....

Thanks for all the ideas. That's what's great about this forum