why disel engines sound so diferent then gas engines

Diesel engines run on the heat of combustion. Gasoline engines require a spark to ignite the fuel. Cetane (diesel fuel) is a thicker fuel, is a slower burn, and contains more BTU's per unit volume than Butane (gasoline). For a diesel engine to reach enough heat combust, the compression ratio in the engine is much higher; for example 30:1 A high performance gasoline engine (Porsche CGT) runs approx 12:1 compression ratio, while most high performance engines are around 10:1 - 11:1. The higher the compression, the higher the likelihood of Knock, or pre-detonation. That is, the gas ignites from the heat of combustion prematurely; adversely affecting the performance. Modern gasoline engines have knock sensors to measure the amount of heat in the cylinder head; at which point, the computer will "pull timing" and reduce the load the engine is allowed to have until it can perform without the danger of Knock again. Turbo and Supercharged engines for the street often run a off-boost compression ratio near 8:1 . On boost, these same engines run the equivilant to somewhere near 14 or 15:1. There are meathods which are employed to keep intake temperatures down; be it through a water to air intercooler in a supercharged car; or an air to air intercooler on turbocharged cars. The sound in a diesel is predominantly the sound of the high compression, rather than that of the hot, and expanding gasses exiting a tuned exhaust system.