Best person I have ever heard purr on a trumpet is John Dangerfield. He tried teaching me at Unicoi. It was very difficult and not sure how to explain it.
He said he figured it out by accident, took him a few years to do it consistently, then a year of practice before he would hunt with it.
Hopefully someone can come along and explain it to you, but it is not easy and some can probably do it much easier than others.
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