if the oil body is different the sound will change...did you use oil of the same viscosity?
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.
Well, the vibrations from the strings would have more room to vibrate in the body, thus producing a deeper sound.
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. ;)
Engine rod? Engine bearing?
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.
It would sound like a v8, maybe a bit deeper.. Import style exhausts are pretty much just glass packs..
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.
IF it is Turbo charged then that would be the turbo.