I did not know it was knife sharpening but we were walking around Pollensa and it got nearer and nearer, it was coming from a car with a speaker on the front (like they use for the elections). We were with friends and we all heard it yet we were in different parts of the town we all thought the same (that it was some sort of piper) until we actually came across the car they seemed to be looking out and up on the roofs of buildings.