Gearbox noisy

Hi, my name is Jamie. I am new to this group. I was lucky enough to be given (by a good friend) a silver 1.3 - 2006 Jimny. Love it. I also love all the other Jimny drivers giving a wave as we pass by. Unfortunately, I am being told the gearbox needs replacing. When it is in 2nd, 3rd and 4th gear it has started making a grinding loud noise but stops when engage the clutch.
My friend informed me he had before, stripped the gearbox and replaced a bearing. I need to get a second opinion really. He also changed the clutch cable quite recently.
I was about to change both the king-pin bearings as they're (on both sides) leaking grease. This I hope will also fix the death wobble when hitting 47 mph.
The only other thing I have replaced was the throttle body housing switch, as she kept dipping below the idle and stalling. I got a £13 off the market switch, as Suzuki wanted £490 for it!
This hasn't worked, keeps stalling. Any suggestions would be so much appreciated. Thanks. 

Welcome to the forum. First off it's notoriously hard to remotely diagnose noises on the internet, however you have said that it has had 1 or more new bearings already and it doesn't make the noise with your foot on the clutch. This half suggests it may be the clutch or associated release bearings etc especially if it wasn't replaced when they had the gearbox out to do the bearings. As for the idle issue, there are 2 electric things on the throttle body the one in line with the throttle cable and butterfly valve that tells the computer the throttle opening value and one on the side that controls the idle valve, this one is held onto a separate cover with pipes going into it. The plastic part is held on with 5 point security fastners so you'll know if you had to buy a driver to fit it. What often causes the problem is that under the plate with the pipes on the air passage gets absolutely filthy with black carbon, it's gross and needs to be cleaned properly.

Lambert, thank you so much for taking the time to reach out and reply. I will look in to this, as I want to keep the Jimny running as it is a lovely little car.

Hello and welcome, what a fantastic gift!