All my cats make that noise. I think its like a greeting noise. And by the way i think Prrrrrp is spelled with 5 r's not 6.(just kidding,That was an good attempt at feline-ese
We have one who does a variation - brrrowmp! It is so clear, I think if I looked hard enough, I could see her lips smacking on the "P" sound. The other three only ever say mwaaaa!