[supereal]

Use a long screw driver or a metal rod to your ear and check each cylinder for noise at idle. If you have a stethoscope, so much the better. If you locate the offending cylinder, you will know it is a bearing. We had one of those in our shop a couple of years ago caused by an improper sleeved engine which allowed coolant into the oil pan which, in turn, shucked the rod bearing for that cylinder. Pulling the engine is always the best way to check lower end problems, and to see if the crank is scored. A knock may be as simple as a bad fan bearing. Pulling the choke out to change the noise is odd. It could be a broken valve spring which you can check by pulling the intake manifold. They can be hard to see, so look carefully. Let us know what you find.