A heater in a parking garage.
This heater has a wonderful rattle to it; I recorded two perspectives and had a hard time shutting off the recorder. So this is both the longest recording I've posted – it was cut down to fit under Smugmug's 10 minute limit – as well as the most complex sound that I've mixed. I've worked in both perspectives, and then added a third track that's had its duration stretched by twenty times and then pitched down to add some additional low end.
I don't usually monitor the audio when I'm recording ambiances, but for capturing specific sounds it made a huge difference. It's something I'll be trying to do more often as the weather improves.