Hissing sound driver's side under hood

[DanK]

I have a 2005 S80 with 84K highway miles. Recently I noticed a hissing sound coming from under the hood, driver’s side, when I step on the gas pedal. Let off the gas, hissing sound stops. Only hisses when about to shift into the next gear. When cruising with no gear shifting, hissing sound is gone. Car (knock on wood) runs great…no loss of power at all. When in neutral, hit the gas, barely hear the hissing sound. I also noticed a significant drop in gas mileage as well. Any insight would be greatly appreciated.