A mix of noise with sliding time-scale/pitch-shift and freeverb applied in audacity.

Approximate way of generating this sound in audacity:

1. Generate or record noise (about 3s)
2. fade in and fade out beginning and end respectively (about 0.5s)
3. add silence (about 3s before and after the sound (so freeverb has room to operate))
4. apply freeverb with default settings or play around
5. reverse
6. repeat 3
7. select the non-silen part (otherwise in the next step you'll be time-shifting silence, which would be a waste)
8. apply sliding time scale/pitch shift with ending time 400% and ending pitch +4 semitones.
9. Normalize (all checkmarks) -0.01
10. slowdown done! to get speedup use reverse!

Protip: assign key shortcuts (I use 4-0) for normalization, fade in, fade out, freeverb, sliding ts/ps generate silence and reverse.

2 spells available as 44100 Hz .flac and .ogg in a 7z file, includes license text.

Preview generated with https://github.com/qubodup/ogatools