my 97 z71 is making a vacum sound from the pasenger side,i thought it was in the heater our fan area,but with me turning fan motor up and down dosent make a differnce.if any oneknows our ideas would be very helpful.Thanks

 

I have the same problem with my 98 k1500. It seems to happen more often when it is cold and when in either accelerate or decelerate. I have narrowed the problem down to my alternator. I dont know if this is the case for you or not. I came to this conclusion one time when it was making that noise and didnt stop. So i popped the hood and poked my head around a little bit. I initially went right for the vacuum lines too but that was not it. I took one of those mechanics stethoscopes or whatever you want to call it and traced that sound right to me alternator. As it warmed up outside I havent heard it since.

 

Check the vent pipe that runs from the passenger's side valve cover into the front of the intake duct near the throttle body.