This is actually pretty easy to troubleshoot, as there are only a few things you need to check.
- Start with the little o-rings that are on the keg posts. If it is dry or cracked, it will cause the problem you are describing. Very easy and cheap to replace. It's usually wise to keep a few extra o-rings on hand for this reason.
- Check the pressure relief valve. Sometimes it doesn't get seated quite properly, and a small amount of CO2 may be escaping. Pull the valve up and reset it, making sure it is seated properly.
- Check the poppet valves. These are visible on the tops of the keg posts, in the center of the post. If the poppet is not flush with the top of the post, it will leak. Sometimes if you just push down on it a little, the spring will reset itself and you'll be fine. Or, you may need to replace it. Again, a fairly inexpensive fix.
- Check the o-ring around the keg lid. Again, if its dry or cracked, you need to replace it. If you've checked all of this and you are still hearing a hiss, check your hose clamps and tighten them down. These things should solve 90% of all possibilities.