Innermittent Hissing sound from passinger side engine when fan is on

What is your question?

Sounds like air purging from air brakes on a big truck. Happens about every 2 minutes but only when using fan and AC. When fan is off there is no noise

What makes this problem better or worse?

Only when fan is on and does not make noise when fan is off

How long has your 2002 GMC Sierra 2500 HD had this problem?

Don't know just got truck