[cdnus]

It would be interesting to find out if this "flutter" is associated to engine displacement & muffler size. Do the members with big displacements encounter this noise (427+) ?
Is it only smaller (408-) that encounter it?
When I had the original VERY restrictive mufflers I did not have any "flutter" but as soon as I installed the S/S 2.5" mufflers I started getting it.
It's almost as if the flow capacity of the muffler is too much for the engine @ lower RPMS & you get a "sucking effect" that causes the fluttering.
I suspect that if I installed 2.5" turnouts the problem would go away--would look rediculous but might solve the problem. (not going to happen)

Not being a engineer & not knowing anything about expansion-contraction-heating-cooling-velocity of exhaust gasses this is my best shot at the problem.

I am sure that someone here is & can answer with a more accurate solution/cause/effect.

Craig