Engine hissing noise?

I have a 2002 Explorer approx 130,000 on it. Just performed a tune up consisting of new plugs, wires, pcv valve, air filter, fuel filter. Now I am noticing a hissing noise when i accelerate, it only is at low rpms' and only lasts about 1 or 2 seconds. Sounds like it's coming from the passenger side compartment somewhere. It sounds like the pressure change when i swithch the thermostat from off to defrost (or some other settings). Noise does not happen when in park, only while driving. Performance does not seem to be affected. Any ideas what this could be? A loose spark plug, new pcv valve..?? I have herd so far it's a leak in the HVAC system but would like confirmation or any further assistance you could offer. greatly appreciated.