Birdpwr

Hello all. I am new to the Mustang scene. I just purchased a 96 GT 5 speed. Miles unknown because the odo is broken (at 140K). Car is unmolested. So the issue is that it starts and idles fine. When under load, it skips as if it has a misfire all through the range. You can tell it is not under full power. If you gas it with the RPMs under 1700(ex: gas it at 25 MPH in 4th), it gives off a popping like a small backfire. I have not done anything to it yet, like change plugs/wires or fluids.
Also something else to note, while idling, I notice like a faint whistling sound...almost as if it were a vacuum but not sure. Any guesses? Thanks.