The exhaust note depends on resonance so is affected by the length of the pipe and rpm and of course the silencer/megaphone/trunpet if any fitted. As we all know valve overlap and pressure pulses in the exhaust result in increased cylinder filling at a specific rpm with the effect reducing and eventually reversed away from that rpm. You have a similar effect on the inlet side of course. Anyway, there is a sweet rpm for any engine which will sound good (depends on your definition of good) and there is another where it sounds pretty rough. An example is the Manx Norton which comes on cam rather sharply at 5,200 rpm and at 3,500 rpm is pretty gutless and sounds dreadful.