Recently put my '51 8N back together after replacing the oil pickup tube, and it's back up and running. However, I now have a new "knocking" noise when I depress the clutch pedal. This is new, never had the noise before. The clutch still works, I can get the tractor in all gears, and the knocking stops if the clutch pedal is not depressed down. Google says it's the throw-out bearing, but I'm curious why it would go bad all of a sudden. Do I need to check any lubrication issues? Anything I can look at without tearing the tractor apart again? Can I keep using the clutch as is? Any thoughts/input would be appreciated.

I agree that it is the throwout bearing. No way to lub it. It will eventually fail. Must be replaced after splitting the tractor.

Thanks @HCooke. When it does fail, how will I know? Will the clutch stop working/not going into gear anymore? Also, how involved is it to split the tractor?

Not a huge job unless you have a loader. If you have a loader it would need to come off first.

I did my 2240 John Deere. It was a big job. Had to drop my loader. Then there was steel hydraulic lines from the pump on the front of the engine to separate.

Good Luck.

Plan on good blocking for the engine end and a good transmission Jack for the transmission so you can roll it apart.

Having split a few Ns, might I make some suggestions. Put an oil catch pan under the trans-engine flanges in case there is some oil in the trans/clutch space.
First, use sturdy floor jacks. Floor jacks can be positioned under both the engine and trans drain plugs. Second, cut a couple wood wedges to fit between the top of the front axle and the support. This will prevent the engine from trying to roll to one side while you move it away from the trans or when rolling the trans away from the engine. Third, I have used a trailer tongue jack with wheel to support the trans while rolling it away. Provides some stability.

Nice picture, Ed.