Love these forums - do not mean to thread jack but I am having the exact same issue with my ESS G1. It also seems to have been getting worse over the last few months (first noticed it a year ago but it was VERY rare - so rare I kinda forgot about it).
Here is a video of my noises - and it comes and goes so I think it is also due to the EVAP system purging.
I am going to try to reduce the spring pressure a little on the BPV as suggested and see. I am pretty sure that I made sure that the line to the BPV is straight off the system with the Tee going back to the car. I do not run a vacuum sensor off this line either as my boost pressure measurement is taken off the back of the plenum (the piece where the hose goes down to the secondary air system).
Worth a try and really nice to know that this tapping noise is not from inside the engine - leaked me out a little for sure!!
Cheers,