I have a 2006 5.4 3 valve and over the past two or three months it started making a hissing sound during light acceleration or holding the accelerator down to keep speed when it makes the hissing sound you can feel a loss of power once the hissing stop the power comes back it does it off and on has nothing to do with air conditioner there is zero freon in it and the compressor is unplugged the hissing sound sounds like it's coming from the intake manifold it does the hissing intermittently only for a few seconds then goes away does this off and on about every 10 to 20 minutes sometimes more frequently when you shut the vehicle off you can hear the same hissing sound any help would be much appreciated I am baffled oh also at an idle does not make the hissing sound but it is running slightly rough at an idle but not extremely noticeable it fires right up has never stalled but I did get a lean code once about a month ago but not since

 

The lean code plus hissing points toward a vacuum leak.

 

Well I was thinking vacuum too but most generally vacuum leaks are constant this one hole sites and it doesn't do it constantly it almost seems like there's a valve or something that's opening and dumping are into the intake

 

Get an UNLIT propane torch and see if the idle changes as you fan it over hoses and the intake.

 

When doing the unlit torch test it doesn't hurt to have the truck hooked to a scanner that can display live data and keep your eyes on the fuel trims,and when they go crazy you've found your leak. A lot of times if you're not hooked to a scanner the computer will react so fast the idle won't change, and you'll miss the leak.

 

okay I went over every inch of vacuum line every nook and cranny on the intake manifold with a propane torch unlit no changing vacuum or idle vacuum pressure at idle is staying pretty consistent between 23.8 and 24 even IN/HG but on the way home last night running I-35 I was running about 90 mph and all of a sudden the truck started shaking horrible vibration from motor check engine light started flashing I let off the accelerator drop speed down to about 70 engine smooth out check engine light stop flashing didn't do it again the rest of the night and hasn't done it today


included as a screenshot of everything that I can check with my setup at idle does everything look in range?

 

Since the CEL was flashing, there is pending/recorded codes. Yank the codes and post them...

 

I scan for codes nothing pending nothing logged no historic log that's why I'm kind of baffled