[Ron61]

I am not exactly sure what you mean by flutter, but you could have a leaking exhaust around one of the header ports or even in the collector area. And no, that would not be a common noise in any Cobra that I have ever been around. Can you tell if it is on just one side or not? With the car warm and at idle go put your hand close to the end of the exhaust pipes and see if you can feel any change from one side to the other such as a miss would cause. I once had a strange noise but it wasn't exhaust as I had crossed two spark plug wires and they just caused enough of a misfire that it sounded funny. When I stuck my hand down by the exhaust pipe ends, I could feel that one side was different and that side had the crossed wires.

Ron