Raptor could be on the right track. Beeping sounds can be made by your motherboard when you have (in the PC Health Status section of the BIOS) your system set to warn you when it's overheating. Most commonly overheating will occur when your PC is stressed. The heat overload is measured at the processor. The beeps are regular, measured beeps, same sound, always the same space apart. Your computer may well be experiencing problems at the same time - this is quite normal when your PC is overheating (and the beeping mechanism itself may cause problems - its not MEANT to be beeping - that's a warning to shutdown and fix the problem).
This idea fits the pattern of your problem - firstly running Sisoft Sandra caused the problem (SS taxes your system considerably), then it appeared when running taxing 3D games, then less taxing games, and disk cleanup...indicating if this is the cause the problem is getting worse. Given your processor is a very high clocked processor, this also fits the pattern.
Either install a temperature monitoring utility (such as Motherboard Monitor) available at
http://mbm.livewiredev.com/download.html to see if you are running too hard, or run your system til it beeps then immediately reboot into the BIOS and check the processor temperature (under PC Health Status). You could also do a manual check (run your PC hard, bring your hand close to but NOT TOUCHING the heat sink on your processor - if it's hot enough to beep, it's hot enough to burn. If via any of these methods you note a temperature that's too high (80+ is very high, 70+ is a worry, <60 is fine (centigrade of course)) then you will need to have your processor heat sink and fan remounted - you may possibly need a new heat sink and fan combo to handle the load if the current one is not up to it. High performance units (eg those made by Thermaltake) are available, at a price.
Beware mounting a heat sink onto a processor can be a delicate process, and the right materials (especially a high performance thermal compund) is necessary. Check out
http://www.arcticsilver.com/instructions.htm for an idea of what is involved. Also note that you must be VERY CAREFUL when mounting the heat sink. It must be clipped onto the processor socket - involving the application of high force (more than enough to crack the processor and/or motherboard) if it is done incorrectly. If you want details on how to do it properly, post back - otherwise I won't go into it quite yet.
