Forgot the gear was down
An Air India crew forgot to retract the gear, and then had to divert because the fuel was getting low. This is despite the fact that the aircraft could not reach cruising altitude, which might have been a clue.
I have noticed more airlines no longer saying 'up, red lights out' and 'down, three greens'. Which may be a contributary factor. As to the noise, you do get used to it. I had to do a gear down ferry once, and after a while the noise sounded normal.
http://timesofindia.indiatimes.com/india/ai-plane-flies-with-wheels-out-forced-to-land-early/articleshow/59747345.cms