I would like to hear recommendations to reduce the ejection port noise on my suppressed 9mm sbr.

Specs:
Qc10 colt pattern lower
Vltor mur upper
Jp bolt
9mm silent captured spring buffer system (off the shelf, std weights and springs)
Qc10 3.5 inch barrel
Silencerco Octane 45
Primary ammunition is Winchester train and defend 147g (train variety)

The weapon is reasonably quiet, but exhibits significant ejection port bark
Function is 100% reliable, 0 malfunctions to date.

I would like to quiet it down more, but obviously not at the expense of reliability. Not sure how much is possible with blowback operation.

I assume my only real option is increasing the buffer weight, but I am open to all options and suggestions.