360 question

If a 360 Modena revs above about 4000 or 5000 RPM. It's getting a louder noise. What happens then in the exhaust. How come it sounds louder. I hear it myself a severtal times. In some videos i've got you can hear it. For example this one made by myself

http://ferrarievents.tripod.com/PCDV0015.AVI

Whp know what happens above that number of RPM?

Do Modenas have a valve in their exhaust? If they do, then that would explain it. It would direct the exhaust through the muffler at lower engine speeds, but when you gas it, the valve would open and the exhaust would bypass the muffler and just go straight out.