Well, I had board that produced the whiney/screech noise so I decided to RMA it. A week or so later, and I have my new replacement. Had to flash the bios to 311 (done straight away), rebiult with win xp64 from scratch. All running fine.
I have however noticed something odd. I'm often getting what appears to be some sort of interference on screen. Most noticable on large sections of plain white colour - looks almost like a horizontal ripple/stripe effect? Seems to tie in with either high cpu or hdd usage (not sure exactly which yet) - which coincidentaly was about the same times I was getting the noise on the other board...?
I'm using a crossfire setup (HIS X1900XT Crossfire card, HIS X1900XT "slave") using the link cable out the back, and a 21" crt connected via a dvi to vga adapter.
Anybody else experience this? I switched crossfire mode off thinking it was that, but still there. May try using the DVI port on the card rather than the crossfire cable and see if that makes any difference.....