I spent about 3 months before the 06 season perfecting the purr on a diaphram. I tried the motorboat thing and it didnt work right for me, same problems as others mentioned.

The trick for me was starting with a single reed and trying to purr like a cat out of the side of my lips. I did this in short bursts until I had it down. It produced a very consisitent sound and was easy to control. After a lot of practice I have moved past the single reed and can purr with almost any diaphram. Good luck.