I had similar symptoms on my D string ... only exception was I could hear an electronic "click" when using the whammy bar sometimes and the "muddy, muffled, lower-volume" sound would clear up OR recur, making me think it was an electrical connection problem. I removed the D-string, carefully raised the piezo from the bridge saddle and burnished (cleaned) the bottom of the piezo metal AND the resting spot on the bridge saddle and VOILA, problem resolved. Enough dust/dirt had settled to make a poor connection. MORAL OF THE STORY: a periodic cleaning of the bottom of your piezo and the bridge saddle when changing strings can make a difference!