I had a similar problem quite some time ago with my truck. Mine was fixed under warranty, but maybe I can point you in the right direction. On my truck, the loud pop noise was coming from the front left hub where the CV connected to hub (here is a picture for referance, barrowed from mjb1032's post)...

See that gear on the shaft, it mates to the gear on the hub (reference picture also from mjb1032's post)

When you put the truck into 4x4, a vacuum system engages the hub's gear to the shafts gear which supplies the power to the front wheel.
On my truck, I had a leak in the vacuum line going to the front left hub (according to Ford) which was causing it to partially engage (the gears would only barely mesh together) which worked fine with no load, but once you put a load on it the gear would pop out of engagement (due to the weak vacuum) causing the loud pop. I hope this could help you out! Also, this is ALL based on what Ford told me was the problem with my truck, I never checked it out for myself because it was under warranty. My truck has not made the loud popping sound since Ford worked on it.

NOTE: credit is due to mjb1032. All pics were from his post. I do not have my own pics due to the repair being done by Ford under warranty.

 

Thank you for the help and the pics! But from my understanding vacuum is held during 2wd, and when you switch to 4wd all vacuum on the hub is released allowing it to engage so I'm not sure how it could be vacuum related but I don't fully understand this system. Mine is only making these POP noises in 4wd.

Since we have some pics up I'd like to ask, in the pic of the hub the gear/teeth I see are of the hub gear correct? Or is that the actuator gear yall were talking about replacing? I'm confused because it sounds like the cv axle teeth stay permanently locked with the teeth I see from his pic of the hub, and the actuator is another gear in there I can't see from the pic or hidden inside that moves to engage or disengage 4wd?

 

disregard those last questions about the actuator and hub gears, i tore into to mine and understand it now. But guess what, my actuator, cv axle, and hub gears look pretty good to me! im fixing to upload pics so i can get others opinions on them.

But back to the problem, i think the noise is coming from my front diff. I jacked up the front end today, and if i spin the driver side tire while the truck is in park and 2wd mode i hear no noises at all and it spins relatively easy, while i am spinning the driver tire the front drive shaft spins so the gears on the driver side of the diff feel fine. Now when i spin the passenger side tire while spinning the drive shaft in the same direction by hand it feels like something is binding up inside of the differential and i hear a slight clunk/pop noise. I can do this by hand spinning it in foward or reverse and hear the noise. Idk if this part is normal either but the front axle moves a good inch or so when i feel it binding, like its forcing something to turn inside the diff and since the diff is resisting it is causing the whole the whole axle to twist that inch or so. Any ideas on this? I was just in the diff case and everything looked great, but maybe something finally broke loose?

 

The cv axle teeth brand new.


Actuator

 

It is hard to tell with the grease but on the tips of the actuator and hub gears there is still the flat crest on the tip of every tooth, for example they look like the teeth on the cv axle but smaller.

 

dang man this is tricky....i guess next step is to pop the front diff cover off and have a peek inside.

 

ya but it sucks because I just did that and put new Royal purple in it $$. Do you think a u joint could cause this loud of a pop/clunk noise? There doesnt seem to be any play in the joint on the front drive shaft but idk if it can still be bad or if a u joint would repeatedly clunk while moving in foward or reverse for a good distance?? And again only makes the noise in 4wd.

 

it could be a u joint, but u have to remove the drive shaft to really check it for play. but again your truck is newer and i just dont see how all this can be happening. especially the fact that all the 4wd parts dont turn in 2wd so there shouldnt be hardly any wear on them.

 

Well I have 81k on the truck now, and I do Offroad alot. But my front drive shaft and axle turn even while in 2wd. Tomorrow I'm replacing the joint and will go from there. I'll let yall know soon as I do.

And I am in matagorda for the summers usually where you can drive on the beach down here, I missed your question in the other post.

 

um somethings definatly messed up with your hubs then cause your front drive shaft and whole 4wd system shouldnt turn in 2wd on the 04 to present f150s they have iwe hubs that disconnect like manuel lockin hubs back in the day. you either have a solinoid or vacum line issue cause your front axle should not turn in 2wd.

 

but yea man let us know what happens.

 

It's not engaged in the hub, it free spins. I've been told that is how it works on these trucks so you can shift on the fly.

 

Sorry bud but you were told wrong, the cv axles and the front end along with front driveshaft are not suppose to spin in 2wd. The solinoid allows vacumm to disengage the hubs and lack of vacum engages the hubs. My grandpas 2005 f150 fx4 has the same system and none of his 4wd parts spin in 2wd. they designed the IWE hubs so that none of the parts spin in 2wd cause theres less wear and tear and less drag.

 

http://www.youtube.com/watch?v=8E3eJdy3RpY
http://www.youtube.com/watch?v=FmmXh9qET9k
heres a few videos to prove it.

 

Hmmmm... I'm on a 90 mile drive at the moment but as soon as I get there ill double check and report back.