It seems like my 4x4 is trying to engage grinding noise

Clicking Noise From Left Front Tire Area - F150online Forums

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Aug 11, 2001 - I have a clicking type noise comming from the left front tire. It does not ... I can really hear it when I'm driving beside a sound wall or the side of a building. ... the sound. (Yeah right!) ... Almost forgot: 1999 Ford F150 XLT Supercab 5.4 4X4. ... My 2000 4X4 had a ting ting noise coming from the left front wheel. Intermitent Grinding Noise on 2004 F150 4x4 - Page 5
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Sounds like the wheel bearing inside the hub is going bad.

The grinding noise you describe is a good indicator of the hubs. You seem like your right on the money with the issue,

G'day mate yes you have a prob and i think it is empty 'transfer' case Toyota had a big prob with transfer case pushing oil into gearbox , result transfer case empty gearbox full. fix is when you get the transfer case off (maybe have stripped and checked ) then drill a small hole in the front of the transfercase at gearbox filler bung height and run the same oil in both if its a manual that is. hope this helps

You say 'new' truck, 09? or new used? I have had 3 Fords, 3 w/ manual trans. case, if you go into 4x4 Low range, you must come to a complete stop before shifting the trans. case to 4x4 mode, and I always put trans (automatic) in neutral before slipping into 4x4, it engages smoothly, no grinding. 4x4 high range can be put in while moving, but I would let off the gas pedal, shift to 4x4 high range, then resume accelerating. If there is any grinding going on doing it as above, something isn't right. Check the trans. case fluid level also, to make sure that's not an issue.

Even though you checked the vacuum lines and found no leaks it does not mean there isn't a problem. The check valve in the line closest to the firewall actuator is probably your problem. Had the same issue replaced this line and solved it. The check valves in the vacuum lines are your best next guess.

Most likely wheel bearing or cv joint issue. look for a ripped open cv boot. I believe that year used cv joints instead of regular u-joints. Hope I helped. Thanks

It does not seem that sufficient diagnosis was done. No, you cant give a specific diagnosis like that without tearing unit down. It's likely that they told you the most common failure and left it at that. Likely there is a problem with your unit...I think this becomes more of a trust issue than a mechanical one. A second opinion from an independent reputable shop may help but either way, you need to have it fixed. In not using it much, it makes me wonder how it got worn.
good luck

Your trouble is most likely not in the transfer case but in the front differential engagement solenoid.

1. I would suggest that you engage your 4WD while you are in full stop and/or safe range of 5mph, however as claimed by the local dealers your vehicle is capable of engaging even while driving. Engage your 4WD while moving only when necessity requires.
2. That clicking noise while turning extreme right or left would be the birfield of your axle and would only mean your front hub is still engage even though you have disengaged transfer case.
Have your front hubs cleaned of old lubricants or grease due to non usage. dont use grease as lubricants, use thick oil such as gear oil or silicone oil. and use your 4WD every now and then.

Do this first then check if you still feel somtehing is loose on your suspension and others.