I love using "brown noise" & "white noise" mp3 files. Some of my favorites are: distant trains, city traffic, farm life, fire places, camp fires, thunderstorms and the usual - but I ALSO love using various sci-fi files from Star Trek (various view screens), Space 1999 (Main Mission ambience) and a few others (I love changing it up) - because I really do not like dead air!
(Yes I still love peace & quiet quite often)
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Right now, my place sounds like the bridge of the USS Enterprise NCC 1701
(I found some 8 hour ambient videos on YT and threw them all together and converted them to an mp3 - it sounds so cool)