URGENT NEED HELP RB20det loud knock

[mini_camper619]

So I've just been finishing up my project car ready for a track day next weekend.
After taking it for a short run down the road I got back to notice a small knock noise. I though nothing of it
due to it dissapearing pretty quickly. Since then I have factored in my boost to 11psi and the noise has gotten a lot worse
Its a very distinct knocking sound very similar to the noise in this video

http://www.youtube.com/watch?v=QrlJMbuTKJg

It is now happening on idle and as the revs pick up its getting louder.
the odd thing is its sometimes not there on idle but regardless when I pick up revs it comes back

Anyway I'm thinking it might be a dirty lifter as the motor has only just gone back in and might have picked something up on the way
I decided to try some lifter cleaner but no luck so from there I tried compression test
All cylinders except for number 2 were 150psi but number 2 was down at 120psi

Still thinking its a lifter could cause low compression but a spun bearing would not change compression
So I've taken off cam covers and checked all lobes, no scoring on any of them
Then tried sticking a feeler gauge in between the lobes and lifter. It wouldn't fit which suggests lifters are ok??

I have just finished dropping the oil and from what I can tell there are no shavings in it.
WTF!

Starting to get a little stumped here

Any help asap would be appreciated pleaseeeeeeee

also anyone know much about bleeding air from lifters?
I have cam covers and everything stripped ready to play with lifters just know nothing about them
Cheers

[mini_camper619]

BUMP BUM BUMP!