Waiting for pushback...was it while the cargo doors were closing after final bags were loaded? If so, it could be the yellow electric pump. You said on the ramp, are you outside then? Maybe the yellow electric pump and the fuel pumps running and creating what sounds like the noise you mention. PTU is a good possibility if the first engine is started, OR after the final engine is shut down, but not with the yellow pump operating at the gate with no engines running, unless just after shutdown of the engines. A zipper though, i can't remember hearing that noise, the PTU sounds like a slow hack saw cutting something, or a "dog barking" as many customers reference to it.
That's all i can think of...