I've been hearing a strange noise for some time now (sounds like an icemaker line..if that makes sense), and I've figured out that it is coming from behind the glovebox near/at the firewall. I looked under the hood in this general area and noticed what looks like a vaccuum reservoir. Do these things go bad? If so what are the symptoms? The noise is only present when the defrost is selected (or in combination floor/defrost), but not when the selector is on floor or vent alone. I can't figure out what this is...any help?

 

in defrost you are using the AC to remove moisture from the air. maybe the drain lines are plugged and you can hear the water draining and sloshing. it may also just be the coolant in the heater core.

 

I don't think it is the drainage issue. It seems more like a circulation issue, you can actually hear the fluid trying to circulate, but it sounds almost constricted. Could this be a sign of a clogged heater core?