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White Noise Pro allows you to build your own relaxing soundscapes or listen to prearranged ones. As you've undoubtedly noted, there are a few apps like this one throughout this gallery, but they all have their own set of sounds, usability factors, and feature sets. Visit the official app link below to read more since I just recently purchased this one and have yet to give more than a couple of (satisfactory) listens.
App Store Link: White Noise Pro ($2.99)