Knocking in front tranfer case - kawasaki 2012 Mule 4010 4x4

Well, it was probably not the starter causing the knock. Does it knock while running? I really need more info.. please repost with more details.

Here is a forum talking about r&r driveshaft.

https://www.polarisatvforums.com/forums/polaris-sportsman/14436-how-remove-front-driveshaft.html

Hit the tool end with a hammer to knock it loose.

I'm in the UK and not familiar with your particular vehicle..
However, the front gearbox as you call it is probably a transfer unit/case. This transfer case takes power from your auto transmission and transfers it to the front wheels.

You can remove the front propshaft (drive shaft) from the transfer case/front differential and make your vehicle 2 wheel drive. This, supposedly, gives you better mpg.

Front prop shaft..
up front near the diff, there is a roll pin that goes through the input shaft to the front diff.. drive that out, then unbolt the four bolts on the bottom of the diff so you can move it forward.
work it forward and get the u-joint off of the diff's input shaft.. when you get that free and out of the way, gently tap on the rear of the shaft to work it off of the transfer case's output shaft.
hows that.. :)

Engine case, or a label somewhere on the front frame member. IT MIGHT be just stamped into the steel some where...sometimes they are a pain to find. pressure wash the entire front end, frame, brackets, etc.. really well, then search with a flashlight!

idle knocks indicate worn pistons or worn gudgeon ( wrist pin) bushes
check the ignition timing as it may be too far advanced for idle causing piston slap

Could be a rod. Not going to be that easy to do. Valve noise is not a knock.

Look into drive system. out put shaft, U joints, tranfer case, differntrals.

valves make a ticking noise, knocking comes from your bottom end (crank bearings etc). try a thicker oil and see if it changes anything