My D2 with ACE used to make the pressure relief noise from new. It was only after about 250,000kms did I notice it had stopped making the noise.
I put it down to the system operating normally when new, but when the hydraulic pump had worn in it no longer generated enough peak pressure.
The ACE system never caused any problems over the 480,000kms I had the D2 for. Was a bit of tech I wished had continued with the D3.

