Transfer Box Issue (I think)

My 2011 jimny has recently developed a problem. I go into 4x4 with no issues and then when I come back out and drive along I get what I can only describe as a low pitch vibrating hum which I believe is coming from the transfer box. I can then go back into 4x4 and back out again (sometimes several times) and eventually the noise clears. It’s as if it has quite come back out fully. The noise is quite loud. 
once out it drives with no issues and when in 4x4 both high and low, no issues. 
I’ve had the car on a lift and checked the obvious, drive shafts, electrics and the box itself which all have a normal appearance and when on the lift going in and out of 4x4 with the engine running turns the correct axils/wheels as it should etc. 

You help would be much appreciated 
bryn. 

Give the transfer an oil change 
And check the drainplug magnet for excessive metal particles 

I’ve found the issue.
It’s a problem with the vacuum hubs. The system has a slight leak in that the hubs go on no problem, but the when it comes out of 4x4 it only takes one hub off and so this creates an imbalance in the front axil/drive and hence the noise when driving along.
Initially no obvious signs in the pipe work as I usually paint it with anti rust but then found a little corrosion which is where the leak comes from..I think.
It is a 2011 Jimny so I’ve decided to go over to the dark side and have ordered some manual hubs.

Bryn