BTW Irish Steve forgive the pedantry but flutter is (basically) defined as vibration of increasing amplitude, leading to failure, so what you witnessed is not flutter.
Ok, so if a 20 cm section of the rear edge of the inner skin of the exhaust moving from side to side at a significant rate and with a significant amplitude is not flutter, what is it? It was a lot more than vibration, and because it was very localised, I'm fairly sure it was an airflow induced issue.
Over time, I suspect that the amplitude would increase, as the strength of the material was weakened.
As far as I was concerned, it looked to me like a spacer or similar had been omitted, or had been lost, and if the area concerned continued to do what ever we want to call the movement, I would be concerned that stress fatigue cracking could become a problem sooner rather than later, given the amplitude and frequency of the vibration.
We're not talking a little bit of movement here, we're talking a high enough frequency that it was not "countable" by a significant level, and the area involved was sufficiently large for it to be very noticable. I've done a lot of flying on similar turbo prop aircraft, and never seen anything like this on other types, and if I can, I do watch what's going on around me. I've flown on 400's before, in similar seats, and not observed this on other aircraft, hence the comments.
This happened some months ago, and at the time, I mentioned it to some friends who are much more closely connected to aviation maintenance type issues, and they were sufficiently concerned at the description I gave them to that they passed it on to "higher channels" that they knew. I'm not a complete layman where some of these issues are concerned, which was why it got mentioned to them, as it was certainly something that appeared to me at least to be unusual.
Don't get me wrong, I'm not taking a dig at FLybe on this, I've never had any issues with them, (as SLF), and their services have always been reliable on the DUB routes I've used, and in a "former" incarnation, they were a nice crowd to do ramp turnrounds for as well .
