Well, the vibrations from the strings would have more room to vibrate in the body, thus producing a deeper sound.
Well, any change to flow of the exhaust gases will change their sound. That said, how much of a change in sound depends on how much you change the flow. Tips, normally go over an exhaust pipe and perhaps extend it a very small amount. It is unlikely this would cause much change...up or down.
what would be an observable change in sound when the frequency changes
Could be the rocker arms in the engine, change your oil and if you ever get into the engine check those little SOBs out. ;)
It would sound like a v8, maybe a bit deeper.. Import style exhausts are pretty much just glass packs..
An engine might sound like a diesel at operating temperature if the engine was a diesel or if there was an issue with the manifold. Sometimes gases escaping the manifold cover could cause the engine to sound different.
Engine rod? Engine bearing?
Faulty engine
Detonation...Needs to be fixed immediately or will eventually destroy engine.
a loose or slipping belt
An observable change in sound when the frequency changes is a difference in pitch. As the frequency increases, the pitch becomes higher, and as the frequency decreases, the pitch becomes lower. This change in pitch is due to the direct relationship between frequency and pitch in sound waves.
a resonator restricts the sound in your exhaust. removing the resonator will give it a deeper sound. i would suggest changing the entire exhaust system and leaving he resonator off which would make a louder/meaner sound. leaving it on would silence the idle but when you rev it really high it would still be loud.