Sorry Bud. There is not enough information in your message to even begin to
guess at the problem. You might start with the year make and model as well
as engine size and type of transmission.. Yes, all that could make a
difference in what the noise might be.  Also, are you certain the noise is
"fssssssssst" and not phsssssffst. That would be a whole different problem.
Probably a vacuum leak somewhere. Check all the vacuum lines for leaks or
loose connections. It's only bad if it causes a drivability problem or some
function is not working properly, or it causes damage to some other
component. Otherwise, just turn up the volume on the radio and no one will
notice the noise.<grin>

Could be the expansion valve in the dash adjust the flow of refrigerant
when the engine speed picks up. If you only hear it when accelerating
with the AC on it is probably either a leak in the vacuum doors in the
dash(if yours are vacuum actuated) or the expansion valve. If you floor
the car, does the output of the air change(i.e. does the air start
coming out of the defroster vents, etc?)

Chris