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Hi again,
Let's try to identify more characteristics of this noise. I have been guessing that the noise is generated when the engine is under load, but I don't know that. So, lets find out. What we are going to do is to run the engine up in rpm to say 3000 or 3500 or so. We are going to do this while parked with the hood open and and brakes applied (Parking and stopping brakes, both); and with the transmission in drive. Do you hear the noise? Do this test with both a cold engine and with a fully warmed engine. If the sound is generated by the stone cold engine (having sat overnight), chances are that if it is a spark knock, it is because of too advanced ignition timing, because the engine hasn't had time to develop any hot spots from carbon deposits or too high a heat range plugs.
Also, try to describe the noise using some other adjectives, if appropriate. Spark knock is usually heard as a pinging, or mild rattle rather than a hard knock.
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