cb32863George, I have a 77 and I have noticed it at times too. Some times under sail, sometimes with the motor. Do you have your CB up? Also, what about weeds on your CB? Weeds seem to have been the cause in my situation but yours may be different.
All I got.
According to Jerry Montgomery, the designer of the Montgomery 15, Montgomery 23, and the Sage 17, flutter can be caused by rounding on the trailing edge of the foils, both the centerboard and rudder.
Try this: take a spoon, and a bit of flat wood that is squared off. Run your kitchen faucet, so that it's a smooth stream of water. Hold the spoon with the back side just touching the water, and note how the spoon gets sucked into the stream, and deflects the stream. Now hold the flat board in the water. See how it just runs down the board and goes straight off the hard edge?
So, now, with your foils, most come with a rounded trailing edge. This can allow the flow of water to adhere around the back curve, and then shed off with vortices on each side, inducing flutter. For his racing days, Jerry says he would fill that rounding, and then sand it off square, with maybe a 1/8" flat across the trailing edge of the foil.
I have observed this with my Harpoon, especially thrumming in the centerboard. And I have considered this modification, but haven't bothered... As for my 192, there are always going to be speeds where I get humming out of the centerboard pendant line. That's just kinda what happens when you trail a 5/16" line behind the foil. Maybe it would be better with 1/8" Dyneema single braid, but who wants to haul up the centerboard using skinny line like that? Some folks say it means their boat is happy to be sailing along well
I have noticed at various times an oscillation in my tiller. The first time was caused by weeds caught on the pendant line. But I think I have experienced it more as the season has gone on, which leads me to believe it's slime growing on the keel causing a bit more turbulence that the rudder is dealing with. And I have had every intention of getting the scrub brush on a stick, and swimming around the keel. But I just haven't managed it yet, and now the water is starting to get chilly, and I've only got a month and a half until haulout, sooo.... Next year, I'm going to look into anti-slime additives for my bottom paint...