Well, if significant flutter on the trailing edge of the exhaust is a symptom of vibration in other areas, then the 400's definitely have a problem.
Glad to say that is not the case, all aircraft have bits that vibrate a lot, and it can concern the layman, but rest assured that this is not a problem. On the contrary if structures such as those weren't allowed to vibrate then there
would be problems!!
Believe me, no engine is free from issues on a new application, there is already a fix in place, it's now about the physical act of implementing it.
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.
MarkD, yes, no need for any extra power on that airframe, it goes like sh*t of a shovel (comparatively) anyway!