I think your guess on imperfect rim surface is right. I had ~the same experience recently, and LBS agreed it was likely imperfect rim surface.
But I was actually able to locate a very low hissing sound from an area of the rim. That area didnt change regardless of many tries of reinstalling.
But I was actually able to locate a very low hissing sound from an area of the rim. That area didnt change regardless of many tries of reinstalling.