I’ve noticed some occasional grinding and it appears that the IWE check valve is shot. I thought I’d go ahead and replace the solenoid as well since the clip that holds it to the bracket is worn. There appears to be a line/hose missing. (This is not the first surprise the truck has thrown at me)
Anyone provide some insight as to what line this is and why it may have been removed? It appears to be the upper line - not the one coming from the check valve.
2017 SCab XL 4x4

 

This is what it's supposed to look like. It appears the line that is missing is the vacuum supply line to the IWE's. Why? No clue. Your IWE's are locked in the engaged position without that line.

 

Are you the original owner?

 

2nd owner
The vehicle was a state vehicle and had most regular service done at the Ford dealership local to where it was assigned.

 

The only thing I can think of as to why that line is missing, may be the IWE's were having problems disengaging, causing grinding, and someone killed any vacuum from getting to the IWE's keeping them engaged, and no more grinding. Their "fix" worked, but that's a rookie way to go about it. Strange.
At any rate, with the IWE's being engaged permanently, it's not likely they are causing your grinding.
Things it could be,
Brakes
Front diff pinion bearing
Wheel bearing.
CV joint
Are you wanting to restore to factory after you find out where the grinding is coming from?

 

As JohnDay in BFE said, with that one hose disconnected, your IWEs are staying engaged full time, and the rattle is likely not them. For a double check, unplug solenoid and drive a bit. If noise still there, my next thought would be wheel bearing. Often they can show there noise at slower drive, and turning left or right (maybe a mall parking lot)

 

Not necessarily interested in restoring to factory if the "fix" is doing it's thing. It does appear that there is a line that has been capped, but I'll have to spend some time tracing it this weekend.

 

The vacuum system disconnect fix cost me about 2 MPG, if you are putting miles on then doing it right will pay for itself in gas in a year or two.

 

If the vacuum line is plugged, where is the plug, at the engine area? Someone doing how they did, had a slight understanding how this thing works, but not totally, or they wouldn't have removed the line they did.
In the photo of my truck, you see the line in place, and see the checkvalve? The other end of that line, attaches to the vacuum source at the engine.
Myself, I'd likely replace the lines that are missing, and go from there, once you find what is grinding. My first suspicion would be the front diff pinion bearing. Your front diff/driveshaft has been turning ever since someone removed the lines. Do not confuse this with driveshaft turning on the 4A, TOD Tcase, normal for that one to turn all the time.