Not exactly a machine, but we have liked the "Happiest Baby on the Block" white noise CD. It lasts about an hour and, of course, has volume control because it's in a CD player.
My first son had a terrible time sleeping but would calm down with water running, so we often put him in his bouncy seat in the bathroom with the tub on full force. I finally got the bright idea to record the tub running and would then play the tape for him. When we moved and no longer had a tape player available, we found this CD. It's been almost six years now and we play it for naps and bed every night! (Though we're no longer using it for the older two, for whom we originally bought it.) It even helps me drown out my DH's snoring (at least a little!).

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