When my truck engine is running my heater makes a constant hissing sound and will only blow out of the...?

Question:
dash vents (it will not blow on my feet or the front defogger). It also hisses when I am off the gas, but the more gas I give the car, the less the heater hisses. This happenes even when I have the heat/AC turned off. Any advice? It is a 2002 GMC Sonoma.

Answers:

Will removing your CAT from your car make it go faster or will you lose low-end power?

Find and fix the vacuum leak. When it fails, it defaults to the defroster vents.

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Car electrical problems?

You have a vacuum leak under the dash that controls the heat and a/c vents you need to get that fixed because it will also affect your computer and gas mileage.

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