2006 F-150, 4x4, 5.4, 30,000 miles

For the last two months there's been a LOUD hissing noise coming from my front end. It sounds like forced air from a compressor. Here are the conditions:

1) Only under load - not Park or Neutral
2) Begins under light throttle in any gear
3) Continues if throttle position is maintained or increased
4) Goes away if I let off the throttle or engage 4WD
5) When switching back to 2WD, noise stays gone as long I stay in 4th gear

I've read a ton of posts regarding "hissing noises" but none of them mention the noise going away in 4WD.

I'm under extended warranty, but I really don't want to deal with the dealer telling it's my tires or some other bs. My truck just turned 30,000 miles.

Any ideas??

 

You have a major vacuum leak in your 4wd system. Vacuum is applied in 2wd to disengage the IWE's, in 4wd there's no vacuum. Take it in on warranty.

 

Makes sense, thanks. I can't get it in to the dealer until next week. Am I doing any damage by driving it until then? Should I leave it in 4WD?

Better question: Since I'm hearing "hissing" and not "grinding", is it a safe bet that there's still enough pressure to keep the axles disengaged, or is the hissing sound actually the grinding of the axles against the hubs that everyone talks about?

 

Headed to the dealer for an "extended warranty" repair tomorrow morning. Stock wheels going back on tonight...pita. I don't think my IWE's are trying to engage yet, so I'm going to take care of it before the grinding starts. After reading for HOURS about how the 4x4 system works, my money is on a leaky bladder in one of the actuators. I think the leak is small enough right now so there's still enough vaccum to keep them out of the hubs. Hopefully I can get out of this with a $100 deductible. I post back.

 

Fixed! Ended up being the check valve between the throttle body and the solenoid, along with a broken vacuum line exiting the solenoid. They also replaced the solenoid. Even though mine already had the water shroud, it wasn't the latest version.

I guess it doesn't always sound like metal-on-metal when the IWEs are trying to engage.