a) Someone thinks a blow dryer is relaxing. b) Someone thinks OTHER PEOPLE think a blow dryer is relaxing. c) Someone takes the time to film a blow dryer in HD video.
There are some good rainforest atmosphere sites on YouTube, although my own experience is that a rainforest can actually be an enervating and very noisy place, so I guess it depends on where you come from (displaced Englishman here). A more obscure sound that makes me feel very peaceful is countryside bird tweets, especially cuckoos, and the far away sound of a hand-pushed mower going back and forth (not motor, or electric!). I have been known to Zen out, too, at the beach, just staring at the waves; but I think that's an ancestral thing. Oh, and car alarms: I love 'em.
Dawn chorus is good, if you're in the right place in relation to the general mix:
Actually it's not as good as once it was, since many birds move north with climate change. But it's amazing in the British Isles and W. Europe. Actually I dunno if it's as much exhilarating as relaxing ... both .... rejuvenating.
Autonomous sensory meridian response (ASMR) is a powerful element in my life. As a kid there were certain sounds that would, for lack of a better description, send me into a state of bliss. This was different from the pleasure caused by music, words used well, or wonderful foods. It was, as described by Wiki:
a brain tingle that was deeply soothing. And entirely non-sexual.
I had no idea there were others who also had this response to certain auditory and visual stimuli until I stumbled across the YouTube community devoted to ASMR. There are so many videos on YouTube that have this weird effect on me -- dozens and dozens.
I also use a sound machine (rain) for sleep.
The most powerful "triggers" for me are sounds of persons drawing, painting, writing, doodling, or sketching.