Help me diagnose this sound

This can be typical of an exhaust leak at the manifold. You might also be able to make the sound by doing a light "brake stand". Hold the brake on, then raise engine speed while in drive. If you hear the noise, you know it's not drive train-related because the car isn't moving.

Sounds like its pinging-does it sound like a bunch of marbles being thrown into a tin can when you're accelerating

You don't typically replace anything except in the rare event it's cracked. Usually it's tighten some bolts, replace a steel gasket, or repair some other leak in the exhaust system. All depends on the cause of the noise.

Rasmataz suggested another possibility with spark knock. That would also only occur during acceleration. The best approach is to have a mechanic listen to the noise and perform an inspection.

Usually it has to be under load for exhaust systems to make a noise. It can be due to increased pressure in the system, the engine is rocking and exposing a leak in the spring-loaded flexible connector, or just because more power comes from more burning fuel and the noise gets louder.

You can also use a helper to make the noise, then listen under the hood with a stethoscope. You can find them at any auto parts store for less than ten bucks.

Besides the noise, there actually is a way a leak can cause problems. Between the pulses of exhaust gas flow, the momentum creates little pulses of vacuum that can draw in outside air through that leak. The oxygen in that air will be detected by the oxygen sensor as a too lean condition. In response, the Engine Computer will request more fuel to go with that air. No matter how much extra fuel it requests, there will still be that unburned oxygen so the computer will keep asking for more and more fuel. The mixture going into the engine will be way too rich, you might smell raw fuel at the tail pipe, but the computer will think the mixture is too lean and it has lost control.

Most of the time that doesn't happen and the engine runs fine. This is one of those scenarios that are the one out of a hundred cars that no mechanic can figure out.