let the weirdness continue

[cavx5]

hi all...new sound emerged from the x....now when the hvac is on regardless if a/c is on or just the fan... when you accelerate you hear a faint constant noise and when you let up off the gas it goes away...if you shut off the hvac their is no noise..the noise sounds like whisling/ hissing noise..that is the best way i can describe it..it is such a weird noise that it is hard to explain..i almost certain that it is not an electrical noise..it only does it while moving because if i rev engine while parked it dosent do it...could it be a pulley or belt...any help...thanks rob