Thanks. This has been a very helpful forum (especially Winston1).
The tractor I purchased had been stored outside for six years and not been used in one year (at least by the owner I bought it from). But, the hour meter works, and only has 148 hours on it (rebuild?). So, my first order of business is to change out all of the fluids and questionable hoses and seals. I drained the fuel which was red (stabilizer?), and refilled it with fresh diesel and a dash of Seafoam. Then I changed the oil which was black and sludge filled (after running some seafoam through it as well).
The result is that the tractor sounds better than when I bought it. It still has a high frequency knock at low rpm, but I have a few more things to do before I post a video of the engine running to get some input on the sound. At higher rpm, it just hums.