olds, are you talking about the external sounds or the internal sounds? i agree the external sounds aren't correct, but i also believe you're not actually aware of what a tank engine actually sounds like in idle (tiger is totally different from a t-34) from the inside or the outside. adding separate sounds the way you're suggesting will do nothing more than complicate things, especially in transitions from idle to full throttle.
pay attention to the existing transition from startup to idle, you will notice there is an overlap that would not be there in reality. that's what would happen if you added an idle sound.