Could unplug the BBV and see if you are getting the same knock first, it will limit the boost to 3-5psi.

Also, 4* isn't that bad either, how long have you been driving the car since you dropped to the 3.4 and put the pcm in? The fuel trims could be still learning themselves and you can be locking the LTFT's in rich/lean causing the car to lean out or dump fuel when in PE or throughout your upper rpm's.