Engine misfire and poor mileage

Shaking is usually a giveaway that a cylinder is misfiring, which would fit with the code (the exact code would be helpful). Usually I would say you need to check the plugs, wires, coils, etc, however the fact you're hearing a noise near the intake could mean you have a problem with your intake manifold, which could also cause misfiring. I would check your plugs, cables, and coils, but I would also check the intake manifolds (assuming a V engine - it is a suburban after all). Look for cracks and check for a good seal, make sure they are torqued down to spec.

You could also have a vacuum leak causing rough running, which could also account for the noise you hear. You need to track down the source of that noise. If the source of the sound isn't easy to track you can use a piece of tubing as a stethoscope.