Ok so I just got my exhaust done. I put headers on and deleted the cats. Now when the car is at idel and I give it a little bit of gas I get a knocking sound from the motor. I know it is not the exhaust banging around. I thought to check my timing. It was at 4 deg with the EST unplugged. It only knocks when the EST is PLUGGED IN. If I unplug it, the knock goes away. So I bumped the timing up and down and again the same, No knock with the EST unplugged. Could me getting headers and deleting the cats mess-up the back pressure and in turn mess-up the computer? Also when I first got the car my old boss tuned it and told me he leaned it out a little. Since then I de-screened the MAF, PP the throttle body, and plenume. Could this have made it run leaner and thats why it's knocking? I thought the o2 would adjust the a/f through the ecm? please help!

 

!UPDATE!

I don't know why I didn't check for truble codes before but I just did and I got code #44 which is....... LEAN EXHAUST ):

So where sould I start to look for solutions.
All I can think of is fuel pressure and vaccum leaks what else?