All right here it is, 01 blazer xtreme. WhenI take off (especally in the summer time after running the ac) you can here a water running sound in the dash kinda behind the center part. It's driving me nuts, andI have no clue what it is or if it is nomal in these trucks. the acand the heat worksgreat with no smell whatsoever.Im not sure what it isI don't know if there is a drain there that is blocked and if there is where it is. I'm almost to the point of takin out the dash to find out what it is ... Anyone got an answer To my problem????

I dont have an answer, but if it was doing that in the winter, comming from behind the glove box, I would bet its your heater core is clogged?

http://www.blazerforum.com/m_200136/tm.htm

Yep. Air in the system.

i have a 97 and it does the exact same thing... i had the heater core replaced thinking that was it but it still makes the same noise.... it drives me insane also but no one that i have talked to knows what to do about it. i have just learned to deal with mine.