This noise when switching strings is a tricky technique to perform cleanly. The way I do it, is very slowly practice the sweep. As your pick moves down to the next string, plam mute the previous string you hit before, e.g if you pick on the 'D' string then as you move down to the 'G' string mute the 'D'. That is the way is the way I do it, I'm not saying that this is the only way to do it or the correct way for everyone to do it but that is the way it works for me. Hope that helps!! :)