Hmmm.. Actually for .mp4 files TiVo uses QuickTime for transcoding, so it can't be a codec issue because, unlike Windows Media Player, QuickTime does all it's decoding using internal code and not third party codecs. When you play it on the PC are you using QuickTime to play it? Or Windows Media Player? If you're using WMP try playing it in QuickTime and see if the static is present. If it is then it's probably using an audio format that is incompatible with QuickTime. QuickTime is a little finicky when it comes to audio/video formats, so if it's using something odd like a lower KHz rating, or VBR MP3 encoding, then QuickTime may choke on it.
If this is the issue then you may want to just try using something Videora to transcode it to MPEG2 first. It has a little better support for odd formats then QuickTime so you probably wont run into the same issue.
Dan