[Zeke]

What happens is there is a harmonic resonance in all systems at some point in the system. The more free flow, the more the sound will really drone at a specific RPM. I used to find that RPM on VW's and the '77 Targa I had, especially in tunnels. But to have that going on all the way down the road at your usual cruising speed would get tiresome.
One of M& K's early models was really loud from 3900 on. That kept me at 65 w/o looking at the speedo. (7:31 R & P)