I use Sleepa.  I've tried a sound machine and a couple different apps.  Most of them have the problem that when the track runs out there is a hitch in the sound that jolts me awake.  I don't have that problem with Sleepa.  With Sleepa, I get to mix a bunch of different sounds together and control the volume of each.  I think each sound has a different sound length, so you don't get that fraction of a second drop of sound that all of the others have.  The only annoying thing is the only way to pay for it is is through Google pay (I refuse to give google my cc) and the free version runs annoying ads (luckily not while sleeping), so I have to start it fast and in the morning click pause and exit really fast, so I don't have to see or listen to the obnoxious advertisements for annoying apps like ticToc (sp?) that I despise.