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.