Help with wind noise and air flow coming from vent

[Chris B.]

I have a 1997 GMC Sierra that I just bought with 75,000 miles on it. When I hit 40mph or faster a whistling noise starts to come from the vents. Any ideas on how to stop it. Also I noticed that air is coming from the vents down by your feet under the dash. The air gets harder the faster you go. If I turn on the fan and set it to blow out through the top tents the air coming through the lower vents stops. As son as I turn off the fan the air coming through the lower vents starts again.
No air should be coming through any of the vents when the fan is off. Any ideas on how to fix this? Many thanks!

 

[IndyIan]

Originally Posted By: Chris B.
I have a 1997 GMC Sierra that I just bought with 75,000 miles on it. When I hit 40mph or faster a whistling noise starts to come from the vents. Any ideas on how to stop it. Also I noticed that air is coming from the vents down by your feet under the dash. The air gets harder the faster you go. If I turn on the fan and set it to blow out through the top tents the air coming through the lower vents stops. As son as I turn off the fan the air coming through the lower vents starts again.
No air should be coming through any of the vents when the fan is off. Any ideas on how to fix this? Many thanks!

Have you had a good look under there? You might have manual vents, which are seperate from the rest of the HVAC system. My 81 Omega had these and I loved them in the summer, makes AC very optional in my climate, which I think is why car manufacturers don't offer them anymore...

 

[barlowc]

Press the recirc button and see that if that stops the flow of air. We had a similar issue with our VUE (minus the whistling noise) and the dealer told me to press the recirc button and that did it.

 

[RoGuE]

I find that all GM cars blow air through the vents (albeit the one you select and not defaulted to the bottom) when the fan is off and outside air is selected. Ford's don't seem to do this (can't speak for any other brand).

 

[mechtech2]

Air almost always comes out of the vents when moving.
This is because the fresh air is 'rammed' in through the intake at the base of the windshield.

 

[Chris B.]

I had a 1997 Tahoe that did not do this so I figured something was not right with the 1997 Sierra. If the recirc button does not fix it what might be the problem and how would I go about fixing it? Thanks!

 

[ltslimjim]

My Stratus, FWIW, made a similar sound, like the sound from the vents when you would typically first turn on the A/C and hear the clutch on the compressor. Hissing almost. Leak in the air system? Not enough freon? Typically would happen more, even on Vent/blower off, if just the windshield vent was selected.

 

[Ramblin Fever]

might have debris blocking closure of the vents

 

[BoomerCTD]

I recall something called Full flow ventilation on GM vehicles.
Air would always come into the cabin.
Mike