Can you post a video of this? Or picture, so I can get an idea of age or maturity? Because chicken sounds are so varied, it's hard to know. How old is he? Do have any other roosters? Have you noticed interesting behaviours associated with the purring? Do your neighbors have chickens? Has he ever crowed?
He may be learning to crow, or his vocal "crowing" box could be damaged, or he could feel threatened and unwilling to express dominance. Any clues would help solve this, but I wouldn't be worried.