Racingjunk Forums - 5.7 liter Vortec Problem

My son has a 1999 Chev Tahoe with a 5.7 Vortec ehgine. There is a slight tapping / hissing noise coming from the left bank of cylinders. The odd thing is....when it's idling and I remove the oil filler cap the engine stumbles a bit and the idle speed drops slightly. I re-install the cap and the idle picks back up and smooths out. I have never encountered this before and think it's probably related to an intake valve (either the spring, pushrod, valve or seat). Does anyone have any ideas what may be wrong? I will pull the valve cover on Sunday but thought someone on here would know. BTW, the tapping noise is very light - not a knock - just a very light hissing tap.