You have a mini Display port on the 2009 iMac.
There are a variety of adapters you can get to send your display to a VGA, DVI, or HDMI source of an external monitor or TV.
The iMac may still be useable with an external monitor even if the built-in display has failed. This will allow you to temporarily use it while backing up files to an external hard drive as you decide on your options for a replacement machine.
The hissing sound is very worrisome, that may be some electrical problem that could possibly start an electrical fire, or some component is on the verge of burning out and possibly may damage other components due to the excessive heat beyond what the heat dissipation system can handle, or which the cooling fans may actually make worse by supplying a steady stream of oxygen. You may notice some kind of electrical burning smell, in that case, don't use the machine and unplug it. You may be able to have a shop open the machine to remove the hard drive so that you can put it in an enclosure or use a bare-drive adapter cable to hook it up to a new computer to migrate your data.