Just a though, maybe the "PAIR valve", see page EG1–96 of a 93 FSM (google it). I know before I took my new to me 3vze of the lot, it had a flutter sound. I found the screws holding the PAIR valve inlet/cover, was missing screw. I installed some that fit, and tighttened the cover/inlet by hand (it is plastic), and no more flutter. The sound reminded me of a piece of plastic stuck in a vacuum cleaner hose.