Speed control capacitors.

[VintageMA]

That's what I was thinking - if you were looking at an AC ripple from the changing voltage of the batteries you would be looking at something way less than 1 Hz.

A ripple from the FETs switching could range anywhere from 8kHz up to around 32kHz or higher depending on the ESC itself. I'm not sure what the MM and the MMM are rated at, but the MGMs are switchable between 8/16/32 kHz to work better with different types of motors.