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Hissing noise when defrost is on

There is a hissing noise coming from passenger side floor board when the defrost is on in the heat setting only. When I turn setting to the vents, the hissing goes away. When the setting is split between defrost and vent, the hissing is still there. Ran the AC and fan through defrost and vents and no noise.

Any ideas what it could be?

Thanks in advanced

That’s just one of the things that happen when you put a 2001 Ford 5.0 into a 1936 Chevy truck and add A/C

Or maybe you drive something else.

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If you have vacuum operated controls on your HVAC system, you have a vacuum leak in the mode door actuator. Since you haven’t told us the year or make, I don’t know if that’s the case.

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