I hear water running under my dash broad every once in a while. Can someone tell me what causes that.
Possibly you are hearing water flowing in to the heater core. If you hear it when you turn the heater on, and at no other time that is almost certainly it
Check your coolant level. Bleed the air out of the cooling system. If you are using coolant, look at the passenger end of the V6 lower intake manifold for a leak. It's a $600-$800 repair that is fairly common with Pontiac V6's that have 60-80K miles.
Search Pontiac Grand Prix Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Pontiac Grand Prix Experts