Why is my engine making a hissing noise on my corsa?

Has it always done this, or has it just began? And I'm going to guess you mean while the car is running, right? The intake on some cars will make a bit of a "hissing" sound as it draws air in, and there are "vacuum hoses" all over the engine, small black rubber hoses that do basically what their name sound like...the opposite of a pressurized hose. If one if disconnected, it may be drawing air in, so look all around the engine compartment where the sound is coming from, and see if you can find a hose disconnected. Be warned, there ARE sometimes hoses that LOOK like vacuum hoses, but are really just vent lines to the transaxle. You can tell if it's a vacuum hose by the sound, and by putting a thumb over the end of it...it should have a bit of suction. Next choices, since you said it was near the fan belt, is that it IS the fan belt, or one of the pulleys has a bearing going bad. In either case, it shouldn't be a problem until it gets really loud, or the belt begins slipping or wearing. Make sure the belt isn't rubbing on something, since that could cause a noise like that. Other option is that if my be the TIMING belt, which I'd guess your engine has if it's anything like the 4-cylinder Ecotec's in the US. That's usually parallel to and behind the fan/accessory belts, but under a plastic cover bolted to the end of the engine. It's quite similar to a fan belt, only tougher and bigger, and it has "teeth" on the inside to ensure it doesn't ever slip, since the alignment is very important...it runs the valves, absolutely essential. It is quieter than a chain or gear drive that does the same thing, but it wears out eventually. Usually they say to replace it every 50,000 miles, but often people don't and are fine...but if it DOES break, the engine will die, and there's a good chance of the entire valvetrain and a piston or two being destroyed totally. I would put you head as far into the engine bay as you feel comfortable doing, and see if you can't locate the sound more precisely...use a home-made "ear trumpet" like a cardboard tube, or even a newspaper or cardboard rolled into a "funnel" shape. Good luck.