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 don't knock at all. When you rev the car up very quickly you don't 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?

Usually the knock comes from where the gas ignites which is in the combustion chamber. I would check your spark plugs clean them then check your ignition coils and lube them up. For the spark plugs check to see if the gap has changed. Also make sure you are using the right gas for your car. You should also use an additive to clean the combustion chamber, pistons, and valves. Use the additive twice in the summer and once in the winter to keep the engine clean from the inside. The additive is inserted in the gas tank.