What is making this sound?

An IR gun is cheaper than pulling the intake off, and will help diagnose if it even needs to be done. Some paint just won't take that heat....... so I could be wrong.

To check the exhaust, the engine needs to be working hard. What I do is braze a ft long length of brakeline into the exhaust pipe, on the engine side of the muffler. Then route a low-pressure gauge up to where you can see it from the driver's seat, and finally, take it for a workout.
Ideally the pressure should not exceed 2psi. If your system hits more than 4psi you want to find and fix the problem.