2WD/4WD problem

Hi! 

I have a Suzuki Jimny, model year 2013. A couple of weeks ago, the 2WD/4WD started to have symptoms. 

When driving with 2WD, there is a wobbly metallic sound from the left front wheel when the speed reaches 40-50 km/h. The sound sounds like metal catching on metal and spinning, as if a "4WD drive lock is dragging." 

The sound stops when I switch on the 4WD and back to 2WD. 

The sound does not return on the same journey, even if you stop. But when I turn off the car even for a moment, the sound comes back when I start driving again. 

4WD and 2WD engage and disengage well. I have verified this with a jack. 

I thought the problem could relate to the vacuum hoses. Some of the metal pipes were rusted. I replaced all the vacuum hoses under the car with new ones. The 4WD generally seems to engage more smoothly than before, but the problem did not go away. I also cleaned the filter that is connected to the solenoids. 

Can anyone advise what to do next? I have been thinking of removing the vacuum hub and seeing what it looks like inside to see if I can find anything obvious. But I don't really know what to do. 

I'd start by taking off the vacuum hubs carefully, and see if one is engaged and the other is free.
If not, I'd put the right one back and drive it to see if the sound is there.

You can go for a quick drive with the hub off as long as you don't encounter a sandstorm or something, oil won't come out, the worry is dirt & grit getting in.

From your description the left hub could be half engaging, which is normally impossible, but it may be seizing on the splines, or you may have a leaky vacuum solenoid valve. You might have the hub on the other side engaged and that one free as an alternative.

If you are careful about removing a hub it will stay in it's engaged/disengaged position when you take it off so you can see what is happening