So, did some reading as I'm not familiar with what you have. You're supposed to be an internal fan... Guessing the one in yours is possibly broke.

Since it's the ISPs device, you could call and say you hear a loud annoying grinding sound like a fan dying (Even though it's probably already dead) and that you want to replacement unit. And then while you have him on the phone double check their current policy on bringing your own modem and ask for a manager to double check for you 'as you don't want this issue to keep happening'