Jody mentioned the PCV valve. Is this what you mean?
The PCV valve screws into the valve cover and has a hose leading from it to the intake manifold. It's purpose is to remove pressurized vapors from the crankcase while the manifold sucks them into the engine with new air to be combusted with the air fuel mixture. There's also a breather hose which runs from the air intake hose to the valve cover which allows fresh air to be sucked into the crankcase. If this system does not operate properly, it can cause excessive crankcase pressure, and encourage leaks and excessive oil consumption.
You'll want to check the PCV hose, breather hose, and PCV valve itself. Check the breather hose to verify it's not kinked or clogged. Check the PCV hose to be sure it's not kinked or clogged, has no leaks, and that it does not suck shut when the engine is running. To check the PCV valve, you should check to make sure air can be sucked in the direction of the intake manifold, but not back into the engine crankcase.