U4s is my favorite iem of all time, but recently I heard a severe drop in quality in the treble (buzzing sound). I know what it is, because it is not the first time! It is the mesh guard blocked up with gunk. Last time I sent it back to 64audio and they fixed by replacing the mesh, but it's barely been a couple of months and it's happening again.
I brushed it as much as possible, nothing worked, I used the pin side of the bush to poke the rim of the mesh, and out of frustration and anger I tore the whole mesh off, leaving an open iem bore. To my surprise, the sound of the iem was crystal clear. Really frustrated ATM considering buying a new iem, but nothing really fits the unique quality that the u4s brings.