This compact machine has six noise modes: White Noise, Thunder, Ocean, Rain, Summer Night, and Brook. You can either have it make the noise continuously until you shut it off, or designate it to make noise for 15, 30, or 60 minutes.
Promising review: "I suffer from insomnia and this has made an incredible difference in the amount of time it takes me to fall asleep (it was about an hour and after a week of use I'm down to about 20 minutes). I also don't wake up as many times during the night. I love how the volume is adjustable and how the machine offers more than just white noise. My favorite sounds are 'rain' and 'summer night.' High quality sound that doesn't sound overly digitized. I've yet to utilize the portability option but I love how that's an added feature! I highly recommend this product even if you're just considering it or if you have doubts. It's changing my sleep schedule for the better!" —Elaine H.
Get it from Amazon for $21.99.