I think it's worth mentioning that when I changed from my maxxis tyres to PR4's I got the exact same noise you describe at about 30mph as well. It seemed to be coming more from the front end of the bike on mine (I have an open face helmet so it makes pinpointing a noise at speed a bit easier).
In the end I put it down to the tread pattern of the front tyre, and it has (I think anyway) gone away as the tyre has worn so i reckon Bbrown has hit the nail on the head with the front tyre tread guess, I'm 99% that's what the chuffing noise was on mine anyway.
In the end I put it down to the tread pattern of the front tyre, and it has (I think anyway) gone away as the tyre has worn so i reckon Bbrown has hit the nail on the head with the front tyre tread guess, I'm 99% that's what the chuffing noise was on mine anyway.