95 3.8L V6 mustang does knock at idle. when applying 3 lbs. vacuum to egr valve it idles the motor down and the knocking stops. We have replaced the egr valve and egr air solenoid. We discovered that the cam sensor was 180 degrees out, corrected that but still continues to knock at idle. We have checked the pistons, lifters, rocker arms, injectors. We have listened with the stethescope and knocking sound can be heard at the intake air box as well as the egr valve which makes it very difficult to pinpoint the exact location of the source of the knocking sound.