There are various noise generators online, however dedicating a whole browser tab to this process seems rather inefficient.
Using sox we can generate a fairly soothing mix of noise:
play -n synth brownnoise synth pinknoise mix synth sine amod 0.03 50
To instead save the output to a file (in this particular case to be 60*10 seconds (10 minutes)):
sox -n output.wav synth 600 brownnoise synth pinknoise mix synth sine amod 0.03 50