Why the pump of coffee makers makes that horrible noise?

[Beta 5]

This type of pump does see use in a lot of small appliances, I've seen them used in smoke/fog machines to pump the smoke fluid into the heating element.
Also seen them in air con units to pump away the condensate from the evaporator.

[Oliver]

I've heard some air cons that make that distinct sound every few minutes while running. I wondered what component would be making that sound until now.