Flashing CEL, Hesitation, Air noise..HELP

First off, no my green plugs are not connected. I have a 99 forester with almost 180k on the od. I have a UEL header and muffler for after mark things I've done to it. I replaced my timing belt, water pump and head gaskets a few months ago, because I found oil in the coolant reservoir and I couldn't drive it a mile without it over heating. The car has been hesitating really bad and nearly dying going uphill. My car smokes excessively as well. I figure I need to replace the valve seals because I found oil on my spark plugs. My cel did come on and was not flashing at one point and i had it checked and got p0420 and p0443. This is just the background...

My problem now is that on a trip to the mtn my cel came on and started to flash. My car then started to sputter and hesitate even more than before. Then I hear a noise that sounded like air. sh-sh-sh-sh. I figure exhaust leak because I look under my car and I see smoke coming from cat-header marriage. I replace the gasket there and the gaskets from my header to the block. I cleaned the surfaces before doing so as well.

The noise is still there and I can't really pinpoint where exactly it is coming from. It doesn't sound like it is coming from the cat but more by the passenger side of the motor towards the front of the engine. I took the plugs out and looked for anything noticeably wrong but nothing jumped out at me...

Any suggestions or info will help. Cheers.