Hub locking by itself??

My 2001 Jimny (lever operated transfer box) started doing a strange thing today... a horrible noise which sounded like a fast rachet/something stuck in brake disc/spline not engaging fully, noise coming from the front offside wheel area.
It's done it twice today, once at around 25mph, then again about 20 miles later when I was doing 40mph on a busy dual carriageway (new pants nearly needed!). Needless to say, I whacked my hazard lights on, slowed down & pulled over pretty damn quickly! 
When pulling off again the noise had gone, but the car felt different - almost like it had gone into 4wd. I turned off the dual carriageway into a side street which was only a short distance up the road and drove up & down it doing a u-turn in junction. Just after the u-turn I heard a clunk noise which sounded like the hub locking/unlocking... is this even possible without me moving the lever? 
This Jimny has been my main daily drive for 14 years, so it's definitely a new issue. 

Any suggestions welcome. 
Thanks in advance,
Elaine. 

Sounds to me like the magnet in the vac hub is not holding it in off position
You can remove the vac hubs and take it for a drive 

I've not had the problem myself but I believe it can be caused by a leaky vacuum solenoid. They are found just below the battery. Anyone that has fitted manual hubs will likely have 2 working spares they could sell you.

Thanks for all of your replies. 
We checked the vacuum filter and all of the connections. The filter had disintegrated, there's now a bit of ITG filter in it's place. 
The vacuum pipes were corroded, so we've cleaned those up and will be replacing them with copper pipes soon. 

The horrendous noise which had happened a couple of times only 20 odd miles apart, might have been a coincidence and totally unrelated...
...the brake shield on the driver's side was badly rusted and we think a large flake of it might have caught the disc. 

The noise hasn't occurred since and it's no longer wanting to go into 4x4 itself.   

Hopefully, I'll get many more years out of my metallic green on/off-road machine.   

Many thanks, again. 

Well, over a month later and it's suddenly started doing it again!
The only way to stop the horrendous noise, is to momentarily put it into 4WD. The noise stops immediately so I put it straight back into RWD and carry on with my journey. It's sometimes doing it 4 or 5 times within a mile now.

On the way home from work this morning I noticed a click sound a few moments before the horrendous noise appears (the horrendous noise sounds similar to when you put a stick in the spokes of a turning bicycle wheel, but much louder & more aggressive). Could it be that the click is a hub (or two) trying to lock? And the front driveshafts attempting to engage is the cause of the horrible noise?

My hubby is going to lock the hubs, to see if that stops the horrible noise. If it doesn't, what else might it be please?

As I said above, it's probably a leaky vacuum solenoid.

Yeah, sorry I forgot to add that Reg was also going to check for a leaky vacuum solenoid... And it seems that may well be the (or part of the) issue. Are new replacements still available?

Yes new are available from Suzuki and used are available from anyone who has fitted manual hubs. Lambert on here will have a couple spare on his new purchase.
An easy way to rule out a leaky solenoid is to pull the hose that feeds the vacuum system off the inlet manifold and blank it of with some tape.