We had a passenger (frequent traveller) a while back, who, on disembarking, described a 'funny noise' during the approach - a sort of intermittent, whining mechanical noise. Nothing seen or heard on the flight deck.
To cut a long story short, a couple of sectors later, one of the hydraulic pumps failed in flight. No big drama, as the aircraft has plenty of redundancy, but it was very useful to have had an initial 'heads up' from the back that something wasn't quite right.
I'd certainly want someone to speak up if they saw or heard something wrong. There's a world of difference between being told how to do your job and being helped to do your job, and I wouldn't dismiss the opinion of everyone in the back simply because they don't have an ATPL.
Who knows, if a few more passengers had spoken out about cracked fuselages or ice-covered 737s and F28s, more people might still be with us . . .