My PC at work was exhibiting some odd behavior; namely it was turning of abruptly. Occasionally when this happened it exhibited a secondary symptom - it emulated the sound of a Boeing 747 at take off.
Turns out that it was caused by "swollen capacitors". I'd never heard of such a thing. Neither had the head of IT who thought the Dell support tech must be drinking on the job when he suggest it. Turns out this is a great description, the tops of several capacitors on the motherboard were noticeably "swollen".
I found a page that talks about this problem at Trend IT.