Gear Problem?

Load weight around 26,000,
While driving downhill or on even, twelve or thirteenth gear, at times I hear the grinding noise. The noise stops when I change to neutral. Otherwise, it will continue to grind and isn't pulling.
And after switching to neutral I can pull back to the gear with no problem.
Now, it's fine. It happened only a few times.
There is a humming noise when I release the accelerator, especially when empty.

Any idea what that might be? It started after driving a few times in heavy snow?

 

The suspension is up. The pillows are good.

 

What was causing it?

 

Just to keep everyone updated.
You know what the issue was. The clutch needed to be regulated only. Less than $100. It solved all the problems.
The original mechanic did work on it but didn't regulate it. He did also other things not necessary charging me extra $5,000. And he doubled the quote for another job.
I'm glad I didn't get back to him because he would lie to me most likely telling me I need a new transmission or repair the current transmission when it was the problem he created working on it. He works by himself and seems to be fishy. He creates problems. He gives you a quote of $2,500 then comes with a total of $4,500.
Pissed I am, I spent thousands in repairs with this guy. I am never going back up there. Initially, I didn't have much problems. It's only after he said that harvesting is finished. Then he was coming up with things. And every time I left a shop, a new thing would show up shortly after. My wife never trusted the guy.
And to think it cost me less than $100 to fix it.