Speakers in base unit

Hi

I was driving about today and stopped for some diesel but when I got back in the car and started the engine the 2 speakers in the base unit in the back were seriously farting. Does anyone know if there is an easy fix or do I need to buy new speakers or is there some other fault that I dont know about.

I've just had another look and its the 2 speakers that sit on top of the subwoofer.

Bad earth?

It has sorted it's self out now so I think I'll leave well enough alone for the moment.