Almost certainly the autopilot disconnect siren. On the B737 it seems to me that it has to go through at least one cycle before quitting no matter how quickly you press the disconnect button a second time. At least on the B757 you only got a 'chirp' out of it if you were quick enough on the second button push.
As for chimes or whoops, not sure if customers get a choice when specifying their order requirements. If I had a choice then I'd go for a full chorus of 'Ride of the Valkyries' when I disconnect.
As for the timing of the autopilot disconnect, that is purely an individual choice subject to SOP's but I think most pilots enjoy a bit of hand flying for the last few miles at least. A non-event and hopefully explained to our SLF who had the courage to come on here and ask the question. Talk about 'lambs to the slaughter' and not even a whimper... I think you're all getting soft in your old age!
(just waiting for the Sun or The News of the World to pick up on this story) 