[LAU] Audacity strange behavior

As a double blind test when seeing what changes were made to Wav files when translating to and from Flac, I also used Audacity to make a direct copy using 'Export'. To my surprise, the copy had a different md5sum signature. Thinking that the export operation might be creating the differences I exported another copy, but this turned out to have a different signature to both the previous ones. A third copy was again different to all the others. Listening to the files I could detect no difference, nor could i see any difference in Audacity, even when stretching the display enough to see the actual waveforms. It occurred to me that the differences might be due to some sort of timestamp embedded into the file header (I don't really know what meta-data Wav files contain) so I looked at the files in a Hex editor, cecking at several identical locations through the files. They all showed up as different, and it wasn't just the same pattern but shifted a few bytes. Can anyone suggest what might be happening here? -- Will J Godfrey http://www.musically.me.uk