Hi, it sounds to me like a over-heating warning. Before spending on HDD I would suggest a good clean-out of all dust etc and make sure that All fans are working, including the fan in the PSU - often overlooked.
Yes before you go spending beer tokens a Good clean up - with vacuum and a soft brush[ girl friend/wife make up brush perhaps?]make sure all is out, Then go rattle all the cards and plugs make sure all are fully home. Of course make sure the power is off and you have equalized your self to the case frame.
If the booting process wont proceed - It JUST COULD be your keyboard. The warning sound may be this. Try gently lifting your keyboard an inch or so and carefully letting it drop back on a hard surface, with a slight jolt. This is a long shot but worth a try.
My brother has said it does sound like a siren. My pc boots fine, but windows freezes once loaded. The beeping/siren doesnt start until after a few minutes or longer. Sometimes it doesn't start until you aks windows to do something. I haven't touched anything recently apart from unplag my other hard drive because i thought it was that. The components are about 4 years old. I've watched the CPU fan, is spinning continuesly without any sticking. Can it be that the CPU and fan are so dusty the fan can't cool it down as well as is should?