[Marshall V. Daut]

If it truly is a deep knock and not a hollow rapping sound (a rod bearing), it's probably the center main bearing, or at least one of the main bearings. Generally, nothing goes wrong with #1 main bearing, often not even requiring adjustment. It's the center and the rear main bearings that cause the knocking.
To test, drain the oil and replace it temporarily with a very heavy oil such as straight 50wt. Buy the cheapest oil you can find because it will only be used for testing and drained afterwards. Start the engine and allow the oil to circulate. Retard the timing as far as possible. If the knock lessens noticeably, one of the main bearings is at fault, usually the center main. Doing this won't affect a rod bearing noise, which can be isolated by shorting out each spark plug one at a time. Sometimes you can short #2 and #3 cylinders at the same time and isolate a center main bearing knock. That's the one that usually goes bad first. The rear main may have too much clearance and allow oil to pass by, but it's the center main that is the noise maker when going bad or out of adjustment. If you are lucky, simply dropping the oil pan and adjusting the main bearings will solve the problem. That way, you can at least see if the Babbitt material is broken when you drop the caps. 'Can't tell about the upper bearing material in the engine block, but as a rule, the Babbitt in the main bearing caps is what goes bad.
Marshall