That is a very common problem with the Altima. Upon taking my car to the dealer, they informed me that me that my vehicle needed to be "burped". I also could hear water rushing behind my dash. The Nissan dealership shop informed me that they fixed it twice however, the problem would come right back. Looking back on postings and other websites, it seems to be a common problem. Not sure what anyone else has done, haven't seen a solution yet. My Altima engine was damaged by the defective catalytic converter and I have been stuck with a payment and no car for a year now because Nissan will not admit that the 2.5 is wacked!