95 Suburban Squeeks or Hisses when in reverse

As the title says, my Suburban makes a repeated noise that sounds like a hiss when I back up. The thing is, it only makes the noise when I'm in the reverse gear. If I leave it in neutral and let it roll down the driveway, it does not make the sound. I originally suspected the brakes but ruled out them out after several inspections and realizing thatit makes the noise even when I'm not stepping on the brakes. I've been told that it's could be either my transmission or perhaps my driveline. I just serviced my transmission today (new filter,gasket,fluid) I'm still getting the noise, but the transmission shifts really smoothly while accelerating now (i can barely feel the shifts) and it changes gear quickly as well with little jerking, so I'm hoping the tranny still has some life in it (265,000 miles).

Any thoughts as to what could be causing my sound would be appreciated.

Might be a dry U joint.

Check the vacuum hoses. The engine/transmission shift slightly one way in drive, and the other way when in reverse