Slight hissing (2 secs) during acceleration

[shadowseven]

Just noticed a slight hissing, about 2 secs, during initial acceleration. sounded like it was coming from the vent area behind the dashboard, sound does not change with vents open or closed. sound only lasts for a couple of seconds and then goes away, occurs at slow or fast speeds. also noticed a 3-4 second hiussing when engine is turned off. also sounds like it is coming from the vents.

Any ideas? thanks.