Weird Knock from Contour SVT?

I have a 98 Ford Contour SVT that has an odd knock. It knocks at different rpms and sometimes it dont knock at all. When you rev the car up very quickly you dont hear it knock, but if you keep the rpms steady it starts to make a knock, and it happens at different rpms, sometimes 2k or 3k. This sound is very nonlinear with the car. It sounds like horses galloping under the intake manifold. Anyone have any ideas on what it could be?

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