OK, if I have 24" of fall over 21' of run how much fall do I need to make a
little noise? My pump is capable of 3300gph at the head it is pushing. Since
I don't have much fall I was hoping for maybe 10 two inch steps at the end
of the run with a few inches of water in the middle. Or should I go for one
fall somewhere in the middle? I was thinking about 12" wide over the fall
area.

I put in a 4200 gph pump that is producing about 3000 gph at my
purifalls. My Stream is about 27' from Purifalls to the pond and I
have an 18" fall from the purifalls(18" wide), a 18" split falls drop,
a 22" fall then finally a 1" - 4" fall into the pond (depending on
evaporation). The fall that creates the best water noise is the
single 22" fall. I channel most of the water to a 6-8" stream that is
a little over an inch deep before going over the falls. I suggest
lining the stream with river rock, especially for where the falls are
as it will reduce the splash zone.

I have placed a small (poor quality) video on my site if you would
like to take a look.

http://members.cox.net/bokowski/falls.html