Hate to say this, but I believe rod knock exhibits this trait. When unplugging the spark plug, there is no explosion in the cylinder, so there is no tremendous force being placed on the piston and the rod, so the noise goes away or at least gets quieter. It is very difficult for one cylinder to get out of time from the rest, because each cylinder's timing is determined by the wires position in the distributor cap. Check the firing order, check the cap for carbon tracks or cracks, check the plug wire. A failing rod bearing will not greatly affect compression, so it is a possibility. If it were debris in the cylinder, it would have continued making noise when the spark plug was disconnected. I hope I am wrong, and that someone else can give you a better diagnosis!! On a side note, many people have the opinion that if it won't start by itself, there is something wrong to begin with, so don't bother using starter fluid, as it will just cause more problems. Its use plays havoc with rod bearings and engine internals if used in excessive quantities. I wouldn't use it at ALL in a gas motor, unless you are planning to rebuild it in the near future, or don't depend on it for anything.
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