Knock in engine of 1940 9N

Here is what my 9N looks like.
http://www.tractorshed.com/cgi-bin/gallery/photo_pic.cgi?pic=http;//www.tractorshed.com/gallery/tphotos/a3245.jpg&firstrec=32&lastrec=47&Parameter=9N&mode=All&what=tphotos

If the "knock" sounds like it's coming from deep inside your engine and has kind of a muffled sound, that's either piston "slap" where your pistons aren't sized correctly for the cylinder and are slapping agaist the sleeve walls, or your un-properly oiled rings have collapsed causing same effect, or your un-properly oiled rod bearings (or perhaps your wrist pins) have ground down and are allowing too much free travel.

It's getting more quiet with use because it's finally getting some oil circulation (albeit poor oil circulation).

Don't you change your oil in your car every X miles or X months whichever is first?

Better change oil with 40wt, put in a new oil filter, and add some MMO and pray you haven't a major problem heading your way (like a thrown rod).

Your N is a low RPM engine that has internal parts that are very, very durable. They are intentionally built heavier then normal to handle the stresses tractor work requires.

To hear a knock (as you describe) in one spells danger. It means some part or parts are broken or have serious damage.

Your tractor is communicating with you. Listen to it.