By default it's normal sounding, which is how it is now
Quieter engines are nearly silent when idling or below safe RPM, while louder engines... get louder at the safe RPM.

But... why?

Mostly because there's small and big engines, but now that I think about it, this might not be a super cool idea to be honest

If it's based on engine size, then I could link it to fuel consumption. The thing is, you generally can hear a lawnmower down the street just as well as a backhoe. Really it's the type of engine more than the size that does it. While I do agree that this would be a neat feature to have, I don't see many people using it , mainly due to MC's low render distance. So I'm going to call it a "would like to do, but feel it won't be used enough to justify the effort" request.