unless you immediately informed the crew of what you saw, you shouldn't be mentioning it now.
In this brave new post 9-11 and low cost short turn round world, it's hard to get to talk to anyone on the aircraft these days, and the chance of getting to talk to the flight deck crew is practically zero. The "friends" I am referring to are both involved directly with the aviation industry, one is a licenced engineer, the other has been involved at a very high level of aviation training on technical and design matters for many years, so this wasn't "idle chit chat"!
On the basis of their evaluation of my report, I reported it to the people (within Flybe) that needed to know, within a few days of the flight.
During the flight, it wasn't a sufficiently serious issue to be causing concern about the safe conduct of that flight, and it wasn't the best of weather, so both cabin crew and flight deck had plenty to occupy them, and there's no easy way to discuss something of this nature without the risk of raising unneccessary worry to other passengers in the immediate vicinity, if they happen to half hear part of the conversation.
I've done enough flying as SLF and pilot that I wasn't concerned for the safe operation of that flight, but I was concerned that if the same symptom continued, there was a risk, as mentioned by MOR, that a problem could occur that would disrupt normal operation.
The fact that I didn't get any form of response was dissapointing, but not altogether unexpected.
It's been mentioned now only because the possibility had been raised
within this thread that there
might be a problem with the 400's with engine vibration.
I made a contribution that I felt was appropriate, and have already done what I felt was needed at the time.
I seem to recall that this is supposedly a discussion forum, and there's no rules that say we can't talk about issues that may be relevant!