After looking at this post:
I had the very same problem with my 3800 2001 Impala. Basically the cat converter plugged up (replaced by dealer) but also burned a hole in my plenum (not covered by GM). So I replaced the plenum (saw the oil in the upper manifold and cleaned it out).
Now once the car warms up I get a crazy squealing noise (I removed the belt to make sure it wasn't the alt, ps, idler, etc). If I remove the oil cap the car stalls and has that crazy suction you talk about. Also if I only remove the dip stick the noise goes away but I can hear tons of air being sucked in by that dipstick tube.
Does this sound like a lower manifold gasket problem? Tonight I took the plenum off and tightened down the manifold bolts after reading your post, but I'm not sure if this Large amount of suction is coming from bad lower manifold gaskets.
Any suggestions?