Exhaust manifolds have a hard life, and their gaskets have it even harder. These thin sheets of metal composite must withstand sustained pressures and massive heat that would wither most engine components. Exhaust manifold gasket leaks can range from the simply irritating to the destructive and can sometimes indicate a more serious problem.
The biggest symptom of an exhaust manifold leak is a pronounced tapping or ticking sound. The volume of this tapping can vary, from barely audible to impossible to ignore, depending on the size and location of the leak. One problem with exhaust manifold tapping is that it's often misdiagnosed as tapping lifters, which sound nearly identical. One way to tell the difference is that exhaust manifolds and gaskets will rapidly expand as they get hot, helping to close the leak and reduce noise. If the tapping noise goes away or gets quieter after the engine warms up, then it's almost certainly the gasket.
If you cannot diagnose the exhaust leak by sound, then you can use the water spray method to diagnose it. A gasket leak that is loud enough to hear will be shooting high pressure gases out like a jet when the engine is cold. There's about a 50/50 chance that the leak is on top of the gasket, in which case it will be easy to find. Simply run a water hose on low over the manifold where it mates to the engine, and watch for bubbling while the engine is running and cold. Do not, under any circumstances, dump cold water on a hot exhaust manifold, or you risk splitting it in two.
Oftentimes, exhaust manifold leaks are caused by a blowout resulting from excess system pressure. This "back pressure" can have many causes but is more than likely due to a catalytic converter that is damaged enough internally to inhibit exhaust flow at high RPM. This happens as a result of running an engine too rich (too much fuel), overheating or using leaded fuel additives. If back pressure is severe enough to cause a gasket blowout, then you've already noticed a severe drop-off in power and possible engine overheating.
This condition will only occur when an exhaust manifold gasket is leaking so badly that it affects all cylinders. It's unlikely, but a loud buzzing noise may indicate that the gasket is completely missing or that your exhaust manifold has actually pulled away from the head. This is definitely a problem that needs to be addressed immediately.