I've been noticing a dull sound coming from G and D strings recently. It sounds normal when I first play it, but after about a set it loses it sustain and starts to sound like a thump. Open strings are fine. I changed to a new set of strings, but it still has the same symptoms. It seems to be related to the change in weather, because in the summer it didn't have this problem. Has anyone ever experience this before and know what might be causing this?