Mazda Trbute ES v6 makes strnage noise when pressing on gas accelerator

Hi

I have a strange problem happening with my Mazda Tribute - when I press down
on my gas pedal, I can occassionally here a hissing sound coming from the
gas pedal vacinity of the car compartment - this can
be heard when depressing the gas pedal at low speed. I asked my local garage
about this and they told me it could be hard to diagnose, but if there
wasn't nothing affecting general operation of the vehicle it should be ok.
Nothing
is affecting driveability of the car. It has roughly 55k on the clock.

If anyone has any ideas of possible cause of this I would appreciate your
thoughts - the cars just come back from a drive on Highway 5 Grapevine here
in LA ! - and that's some push just to get up that hill.

Any help appreciated.

thanks

Brian