Save typing my own answer here is someone elses who can put in words better than i can
though tit easier to give examples and this thread just so happens to explain it http://www.350z-tech.com/forums/153-intake-exhaust/39837-raspy-exhaust-myths-facts-theories.html
to take extracts from said thread
by playing with different parts shows that it is exhaust desing that causes rasp, get the design wrong, you get rasp, get it right, you don;t
again another example of what causes noise in the exhaust.