Glad you nailed it. I got to see a pretty dramatic example of what happens when you dont find the source of flutter. A guy I fly slope with has a homebrew 100" pattern plane thats been converted to a sloper, and it...flew, past tense, pretty well. He was in a mild descent today out at Tick Pt in Marin, when suddenly the elevator fluttered violently, and half the stab blew off. What was left of the elevator became jammed, and the plane bunted from about 200' above the hill to about 100' below the hill, and hit the only rock out there in the shoulder-deep coyote brush. It went in inverted at about 70 mph. This had happened previously with less dramatic effect, so, bottom line, the airplane isnt fooling around when it makes that sound.