sounds kinda like a school bus
 

I have a `95 Disco - here's the problem, it is real unresponsive power wise, and it sounds like a school bus. It doesn't shake or rattle, and the rpms stay level. It just sounds terrible. I've replaced the spark plugs, wires, distributor cap and rotor arm - still the same old noise. Can anyone lend some advice or help? Thanks guys.