Odd 4wd engagement issue

Hi

Appreciate "its makes a funny noise" is always vague, but wondering if anyone has had anything similar.

2009 Jimny. Electric 4wd and xfer box switches.

I have been getting intermittent odd noises, like a badly worn hub bearing rumbling, when traveling above 20mph on tarmac in a straight line. I've worked out I can stop it by slowing down, selecting 4wd, and then going back to 2wd. I am in and out of 4wd a lot so it made sense perhaps I was getting one of the hubs stuck in locked mode. However would that make a loud rumbling noise?

I never get anything out of the ordinary with the 4wd lights flashing. And I can hear the hubs switching. The 4wd always feel like its working when I have it selected and am in slippery situations.

However, recently I have been resolving my noise issue by switching to 4wd and back to 2wd. But then having parked in 2wd and setting off again the noise is back. Switching to 4wd to 2wd again resolves it. My understanding is that if the hubs are on or off, it needs a change in vacuum direction to switch the hub position. Is there any way that parking for a short time would cause this?

I know I need to have a good look at the pipes and see if I can see an issue. I just cant make sense of the symptoms at present. Any ideas very welcome.

Thanks

2 things.
Firstly it could be the collar inside the hub has detached from the diaphragm part that is moved back and forwards. It's held in by a circlip style ring that locates in a folded steel piece. The folded steel wears allowing the clip to moved past it This will mean the hub makes the unlock sound but the collar stays engaged with the driveshaft. The only way to check is to remove the hub and see if the collat is loose.
Secondly it could be a leaky vacuum solenoid. These are found below the battery. If the solenoid is leaking it will slowly move the hub into the engage position. To check this swap the solenoids around. This will reverse their roles, so the leaky solenoid will tend to hold the hub in the unlocked position rather than try to engage it.

Thanks for this. Was hoping to have had a look by now and report back but yet to correspond time off with it not pssing down. Gives me something to look at though.