I just restring-ed my classical guitar. It was fine before the restringing. But now when I make a rest-stroke, besides the normal produced sound, there is another noise entailed. The noise sounds very suggestive that it is produced by something vibrating. This only happens to the 6th, 5th, 3rd and 2nd strings. The problem also disappears when I use a free-stroke.
Any suggestion on the cause of the problem, and how to solve it? Somehow I feel this is a very common problem, and I must have faced it in the past but I cannot remember how to deal with it, or it just vanishes by itself after a while.