Why has iTunes started adding hiss to imported WAVs?

For several weeks I've been experimenting with importing <30-second sound files into iTunes and creating custom ringtones. I've used everything from song segments to clips from freesound.org. Recently I extracted 29 seconds of quiet guitar strumming (with Audacity) as a WAV file. When I play the WAV with Windows Media Player, it sounds completely clean. I went into iTunes and did what I always do for ringtones; set it to import as AAC (first step in the process). But what I noticed this time was that iTunes added obvous white noise/hiss to the music. I tried both the AAC iTunes Plus setting and an AAC Custom setting at 320kbps. Same hiss in both cases. Anyone have a clue why this would start happening?