Ocing my 4850, various problems

Problem one. When I run furmark, I hear a wierd hissing noise that seems to be originating from my GPU/the area around it. This noise stops promptly when I quit furmark, and starts again when I start it up again. I'm running the GPU right now on 2 22" monitors, I have it clocked a 670 core 1000 mem. It's watercooled with the swiftecn mcw60 (I think are the numbers, offhand).

Problem one. What is the hissing noise, and does anyone else hear it? Chances are you wouldn't with a closed case, or if there was a fan over it, so I'm speaking to a watercooled person here.

Secondly, at only these clocks, I start getting VERY mild snow, as in, one white blip every minute or so, but at any higher, the computer locks up. Any advice?

I'm using the amd gputool as my oc tool and furmark as stated for my testing.

1) Yes, my 3870x2 did the same thing, it is normal
2) Sounds like the core is either overheating or overclocked too high

THat's what I don't get, because I"m running a technically high end cooling on the core, and people running stock are getting higher clocks than this.