Every engine that runs on gasoline has an intake manifold, which is designed to aid in the distribution of the fuel and air mixture to the combustion chambers. When you peer under the hood of most cars, you will see a set of large metal tubes connected to the side of the engine block. These are the intake manifold runners, and each one is connected to a single cylinder. The manifold consists of these runners and other pipes that link the air intake valve to the engine.
In older vehicles that use carburetors, the manifold directs air entering the engine to the carburetor, where it is mixed with fuel, and from there the mixture is distributed to the cylinders. In the case of fuel-injected engines, gasoline is sprayed into the intake manifold runner just before the air enters the cylinder head.
The most common problem associated with the intake manifold is leakage. When this occurs, it can throw off the ratio of air and fuel and negatively impact your car's performance. If the system springs a leak, you may hear a hissing noise emanating from under the hood when the engine is running as the air is escaping.
A leaky intake manifold will result in a loss of power output, since the ratio of the mixture will be imbalanced. You may notice your car has trouble accelerating or maintaining speeds as a result of this imbalance. If the leak is toward the bottom of the system, air won't be the only thing to leak out. Since there is less air, the amount of gasoline being sprayed into the manifold will be more than the air can handle, and some of it will settle on the bottom of the pipes and leak out. In this case, the sound you hear may have a gurgling quality to it, which is likely a combination of air and fuel escaping from the manifold.
If you notice a pool of gasoline under your car, you can determine whether it came from the manifold by looking to see if there is water in the fuel. In liquid form, water and oil will separate, making this relatively easy to check. Signs of water indicate the leak is most likely in the intake manifold, but if you don't see any then it could be coming from a number of other locations.