Gm actually has a bulletin regarding this problem. This could be numerouse items causing for this problem, but here is where you should be starting to diagnose this problem.
This may be the result of:
a PCV fresh air hose that is pinched or restricted by plastic casting flash (this would normally be a concern since new, or shortly after engine repairs).
over-aggressive valve lifters (this could develop at any mileage).
plugged PCV baffle drain holes in the valve cover that has the PCV vacuum pipe attached to it (this would normally be a concern at high mileage).
an improperly sealed PCV valve cover baffle (this would normally be a concern since new, or shortly after valve cover replacement).
Important: If Step 3 or 4 leads to valve cover replacement, perform Step 4 on the replacement valve cover before installing it.