Vacuum leak in crankcase

I have a 200 impala with the 3800 series II. I noticed a high pitched squeeling noise at idle, and also when the oil fill cap is removed there is a lot of vucuum pulling in on the valve covers....so much that if you pull it off too fast the engine kills. I took off the upper intake and found a lower intake bolt completely off so I took off the lower intake and replaced the gasket along with putting thread lock on the bolts and replaced the upper intake gasket. I also replaced the pcv valve. After all this there is no squeel but still the vacuum leak. does anyone have any idea what would cause this. I will take any suggestions at this point.
Thanxs