My first indication of a fairly serious hydraulic problem was passengers noticing a ‘funny noise’. Two passengers, on two consecutive sectors, which was enough for us to call the engineers out.
The funny noise turned out to be an engine driven hydraulic pump which was on its last legs, and eventually ate itself a couple of sectors later. No noise or any other hint of a problem on the flight deck until it went bang.
Moral of the story – if anyone spots anything that doesn’t look or sound right, I’d like to know asap.