Disable? If it can't advance the spark to catch up with speed, It will be "late" and that would cause knocking. Make sure vacuum diaphragm is OK and vac source is reliable. Apply suction to hose, see if it moves lever and holds. A new exhaust means it could be leaking. You have to always recheck manifold bolts after several heats and cools, especially when new, and new gaskets. Problem with broom or stethoscope is 1800 is way too fast for that engine to do at rest without load, I believe? Did you say that it does the sound at fast idle or under load (driving)? How do you know it is 1800 RPM? tach? It is possible that it is spark knock or exhaust leak somewhere at some connection. I am sticking with those until you do a "rod bearing test" as described, which I know nothing about. I do not recommend too much driving in case it is internal components. Maybe someone will advise when to tear down. I guess you don't have a little dieseling do you? Run on after key is off? That would be caused by timing issue or exhaust leak, believe it or not.
