OT Brainstorming... Acoustics/harmonics

I've just started training to take over a position in the NVH engineering department, and one of the topics in my training today was brainstorming a list of causes for 2nd harmonic noise in small DC motors (those used for window regulators in automobiles).

Does anyone have any insight they could offer? I'm just beginning to wrap my mind around this stuff. The 2nd harmonic is around 250Hz. We're thinking it's a resonance, but not sure the cause.

Thanks in advance!

Keep going.
what is the motor connected to?
what components?
gear box, wire spool, wire, bearings, window, window track...…
is this in all units?

have you tested the motor by itself, no gearbox, just the motor?
food for thought.
james

Motor is being tested as part of a molded door module, but we're hoping to get more data from tests of the motor alone. The system in question is in Europe and we're supporting remotely. I'll follow up with more information when I get more.

I was asking also for just general ideas of what can cause noise around that frequency, as it seems that each harmonic has a sort of "usual suspects" from my extremely limited understanding.