Here we go again, a new noise to figure out. 2011 5.0. Accelerating up hill I get what sounds like an air leak, or hissing sound. No loss of power or any other problems. Letting off the gas and it goes away. Step back on the gas and it may or may not start again. Checking under the hood I see no leaks or anything unusual.

 

I get the same on mine however, I've chalked it up to normal operation to be honest since nothing else is out of the ordinary. I also narrowed mine down to the HVAC system making the noise since if I turn off the HVAC while it's "hissing", it stops immediately. Being that there are no other issues, I've never cared enough to look further into it and it's been 70K miles of driving.

 

I know for a fact I get it with the AC off. Not sure if the fan was running though. I'll have to check that out. Thanks.