First of all this will seem like the blind leading the blind because I cannot purr very well on a mouth call, although I might be able to help you a little!!lol. First off, you want to try and do it on the right call, and although many guys can purr on about anything, you will need more help from the call to begin. Get you a call that is a "batwing" cut. Its bascially two half moons that come to a point in the middle of the top reed. Take both hands and draw it like i described if that helps visualize. To purr, it is basically making the noise like rolling the "r' in spanish.

Your best bet is just to get the right call and then just play with it enough to start to purr. Alot of call makers will build and cut calls like you want them so call one or 2 up and tell them your trouble and get them to send you something to try. Good luck...