I am 100% sure of the cause of the siren problem - for me anyway: CPU overheating. While that sounds obvious, it was an arcane issue with me. I always use a temp probe on my CPU. I tape the tip to touch the edge of the core. While the tip of the probe is flat, it is shrinkwrapped about 1" down, which makes it thicker here. This part sits on the PCB of the CPU. This section was preventing my heatsink from seating properly on the core.
The tipoff was when I stuck in my 8RDA+ board, and I noticed temps soaring to 59-60 C watercooled (with water at 25.5 C). 3DMark would lock up even though Memtest was clean, and I could not get my XP2100 above 2200 mhz. I had also seen high temps earlier on the NF7-S. I figured oh well, AMD chips run hot. As a test, I pulled the probe off, and no problems with the siren since, on two NF7-S boards and two CPUs which gave me tons of problems earlier. On one (not-too-good) XP2100 I got to 2455 mhz at 1.98 volts. Heh, if it hasn't sirened yet, it ain't gonna happen.
This shutdown system is a very effective safeguard, as I've experienced the problem maybe two dozen times overall, yet I have not fried a chip or burn't out a board. If you get a steady beep after exiting the BIOS, that's usually the ram.
So if you are having the problem, check to see if your heatsink is seated properly, or get better cooling (though it would have to be pretty bad for this to happen with the sink seated properly). I imagine the stock sink with a real dud of a chip + high volts might cause this to happen.
Bending the probe up probably will help a bit to get the board to read accurate temps. More than likely, you're simply getting better seating of the sink on the reinstall. I don't think it's critical to get the probe to touch the back of the CPU. It's probably independent of any software (i.e. HW Monitor) or BIOS linkage. It delivers dead accurate temps to the board to tell it to shutdown. It just needs to be near the CPU or probably reads air temps in that pocket. And don't trust temps in the BIOS or Windows software! When I saw real high CPU temps of 59-60 C via my probe, the BIOS and AIDA temps read OK!
BTW, doing back-to-back testing, the 8RDA+ is overclocking a tad better than the NF7-S. I got that XP2100 to 2455 using a tad less VCORE. But I max out at 221 fsb on the 8RDA+ versus 223 on the NF7-S, with my 2x512 XMS3500 at 2-2-2-5 with CPC enabled. These are unmodded boards with factory BIOSs (I'm doing baselines before I try mods). When overheated, the 8RDA+ doesn't beep (it doesn't use a probe). It will just freeze on boot.