Miss Fire

Hi, i'm new to the forum but seeked help over Google when eventually becoming fed up of the strange problem my Fiesta seems to be having. The car is a 1.25 ltr Encore 1997 model and has only developed the problem within the last month. The problem seems to be that whenever i accelerate by pressing down the gas pedal the car seems to make an audible hissing sound, almost like air is escaping or being sucked into the car.

The sound seems to originate from the rear left of the car, where the fuel cap is. I've already tried driving around shortly with the cap off to see if the seal was somehow causing the noise and have also recently changed both the oil and air filter, but still the noise seems to occur. I'm not quite sure if there is a large performance decrease, i'm averaging about 330 miles to a £35 tank of petrol. I'm only a 17 year old new driver with little, but increasing, knowledge of cars (however my step dad and best mate are both fluent). The strange thing is that when revving at a stand still, no hissing or sucking sound can be heard, but when moving even in first or second gear it occurs. However, when i shift into 5th and lightly tap the pedal the sound disapears so it is definitley related to the revvs.

Finally, it does seem like the sound can be heard from outside, almost over the engine sound, and sounds ALMOST like a turbo whistle but all the time.

Any ideas?

Thanks =)