Interesting. I watched the youtube vid and to me it sounds like a piece of sheet metal or something metal vibrating, not anything related to the front axle, especially since the tractor was not moving. I have herd something similar on my BX. It only happens when the rpm is in a narrow range and even then not all of the time. Months back I thought I had isolated the noise to a piece of sheet metal at the lower front of the front grill area. On my BX, the sheet metal "cover" is attached on one side but not the other. It must be intended for removal for service I assume. At one point when it was making noise, I could press on the "loose" side of the panel and the noise would stop. But, I then added a bolt on that side, but only at one corner. After that the noise subsided for a while, and now I hear it again. I'm thinking I may need to secure it better, or add some felt padding between the edge of the sheet metal panel and the frame, which is where I think it's vibrating. Now, of course, this is a BX and no a B, but maybe Kubota uses a similar piece of sheet metal on the lower front side of the tractor.
In the attached pic, it's item 140. Shows it bolted at the top of each corner, but I don't recall a bolt on the left side. Anyway, you can see where the bottom right or left side of the sheet metal could vibrate against the vertical strut. Anyway, something to look for on your B's. See if you have a similar piece of sheet metal and if it's loose at one or more of the corners.