ORIGINAL: quiksilver
Purring with a mouth call is the second hardest call to master (second to the gobble).
Instead of doing the R-R-R-R-R sound, (and I'm doing it with no call in my mouth right now), do a Whd-d-d-d-dsound. You'll have to sorta cup the back of your tongue, because the rolling "R" sound pushes the call too hard into the roof of your mouth.
i had never been able do the purr until this year. i just tried to make the sound and finally found a way to do it. even though i don't know how to explain it quiksilvers explaination sounds about right.