Robert,
Seafoam can't hurt, and it's great for cleaning out deposits, especially if you let a vacuum hose (like your brake booster line) suck it straight out of the can into the intake, shut down to let it soak a while and then start it up again to purge. But I can't see how it's going to cure the knock you've described. What's strange to me is that while you say you hear it at lower rpm's, it is not there at idle? That's kind of strange for an internal knock, at least in my mind. I like the idea of getting it in the rpm range where the knock is worst, having someone hold it steady at that rpm, and then use a big screwdriver, metal rod, or even a hardwood dowel as a stethoscope, moving one end around on different parts of the engine (stay clear of belts & fan) with an ear on the other end, trying to pinpoint where the noise is coming from. If it is indeed only happening in a rpm band above idle, but below say 2000rpm then Charlie may have nailed it with a cracked flex plate, or robot's timing chain slap? Whatever, let us know what you find!!