Re: Hissing sounds
Cap explosions are rare, but they have happened. If it's leaking, that's just a sign that the shell is cracked or has a hole somewhere, so an explosion would be somewhat unlikely. A good idea would be to disconnect power to everything in the system (HDD, CDROM, the fans other than the CPU fan) and then turning the system on to see if you can isolate it better. I'd turn it on like that for a good 5 seconds or so at most to get an idea of where its coming from, then replace the video card with another one, or leave it out if you can use onboard, and try again.
Alternatively, you can assemble the PC on a flat surface - a solid wood desk will work fine, use the motherboard's box to support the board if you're nervous and then power on the system so that you can listen to something more closely. I do that all the time, haven't fried anything.