2000 Mercury Sable weird noise behind glove box when accele

Looking at purchasing a 2000 Mercury Sable. Is is GREAT shape. Only thing that bothers me is when I'm accelerating there is a "bubbling"/"cycling" noise comming from behind the glove box. The dealer said it was a common problem with that year model, and had been checked by a professional. He said it was because of the way the water system is set up, and that they later changed it in Taurus/Sable. I was wanting to know if anyone has heard of this, and if it is fixable, or if it is nothing to worry about. Thanks