Knock knock knock

D7 17A C Model - went from purring like a kitten to a very distinct, impossible to ignore, knock in about 10 short minutes.
First I wondered "am I hearing things", but in another 5 minutes, I knew what I was hearing. Low RPM, no knock at all, by
quarter throttle, I can hear it, and I chickened out at half throttle because it really starts to pound. Sounds to me like it's a
rod bearing. I did the old farmer test - took a screwdriver and put it up to my ear and listened and I can't tell any difference
in any of the 4 cylinders, which leads me to believe it's on the bottom end. There's no blow by and it carries the exact same
oil pressure, even at an idle, as it did before. The motor is in excellent shape - takes no oil, starts and runs great. Is there
anything else "down there" that would sound that way and is there any way we can do a pre-op test to see what we are up
against? Please tell me that if it is a rod, it has not ruined the crank shaft. Thanks.

Check fuel injectors, a bad injector is known to sound just like a rod knock. Loosen the fuel line to the injectors, one at a time and see if it goes away.

Had the same problem with a D7 17A several years ago and after checking all of the rod bearings the problem was found to be a bad injector pump. Like you I thought it was a rod knock. Bought a new injector pump and tried it in place of each injector pump. Found the bad pump after the fourth try and when I replaced that bad injector pump the knock went away. Good luck. Try the replacement injector pump before going to the trouble with looking at the rod bearings.