Please email Stan - I do not have the issue with my BluRays at home or at the factory - so not sure what is happening in your setup.
I have the same issue, but it is not every time. My theory is that it is caused when the BR disk switches codecs. For example, if the previews are in Dolby Digital, but the main movie is in DTS Master HD, the SP3 needs to relock. When that happens, the pop or screeching sound is produced.
The problem is better now than it used to be in earlier versions of the firmware. However, it still happens frequently enough to be worrisome that it will damage a driver on my speakers. Think what improved it a few versions ago is that a short mute was introduced in the firmware, when the codec changes. However, I think the mute only happens when there is a gap between the two codecs. Where I notice the issue the most is with my HTPC. If I am watching streaming video with Dolby Digital then switch straight to Dolby Digital+, I get the screech. On the other hand, if I switch from the HTCP to a DVR, there is a break in the sent codec, so no screech.
Hope this additional info from a different owner helps.