Originally posted here by The3ntropy
If it is a siren type sound, or long persistant beeping, you were just saying that there are options in the BIOS about sensors on the motherboard, you said something about a fan sensor. There is a good chance that there is a thermal sensor(s) somewhere else on the board, a possibility is that something is overheating, CPU or chafe, and that the motherboard is alerting you of this. Check for something about max operating temperature, or something similar. If this is the case, simply up it 5 or 10 degrees.
As far as I know, most PC BIOS' still do indicate error conditions by the beep "pattern" during boot up... a single beep is normal, where-as something other than that, such as a long beep or multiples (or various short/longs) mean different things - often calling the BIOS manufacturer and giving them the beep pattern, they can tell you what's happening... of course, there's always the "remove everything and slowly add things back in until the problem repeats" sort of troubleshooting approach, too (well, you get the picture I hope).

But, in any case, if you're getting a heat issue... my personal recommendation would be to not simply up the threshold on the temperature - perhaps as a last resort or to give you a little more time to troubleshoot it after bootup... but generally, you'd ideally like to figure out how to bring the temp back down -- look at the airflow in the case, possibly adding another CPU fan, etc. There are lots of ways around the problem, but, in my opinion, simply up'ing the alarm threshold can be asking for more problems further down the line and should be done as a last resort (remember that the defaults were burned in to the BIOS by the manufacturer... people who, in-thoery, know more about the board than we do... I wouldn't muck with settings where you might not like the results of "something bad" happening... hehe)

Just my $0.02...