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.