I've been through a 4 month ordeal with Sony trying to get to the bottom of this. Here's the story. I bought an MZ-N1. One day after the 30 day money back period, I did a recording trying to capture some fairly low level audio with mics (long cables... lots of isolaton between mics and MZ-N1). On playback I noticed the sound of a faint motor scooter or chainsaw or something in the background that lasted about 10 seconds and recurred about every 106 seconds. This recording was done using the internal battery. I did further testing and when the power ran out, I started using the AC power supply. With the AC power supply, the noise remains constant throughout the entire recording after it starts up about 8 seconds into the recording. The noise is predominantly in the left channel. Sony has had this unit for 3 of the 4 months I've owned it... it's been sent to them twice and 4 separate facilities have looked at it. Now they're trying to tell me the noise is "normal" and nothing is wrong with the unit. You can see a screen grab of the waveform of this noise and the conditions necessary to reproduce it and download an audio example in MP3 format at http://www.mountainlake.org/noise
I have tried this over and over using several different sets of quality mics... different locations, etc. The results are absolutely repeatable and predictable. I've also found that I can record the noise with no input what-so-ever. No mics..... nothing.
If any MZ-N1 owners can take a couple of minutes and visit the URL I posted and try the simple test instructions I posted at the bottom of the page, I would greatly appreciate it. If you post your results here I can point Sony in this direction and, hopefully, refute their contention that this noise is "normal". I already know that all of you will not find this noise in your machines because the noise is NOT normal... it is ludicrous.