It is confirmed that the sound came from Hydrostatic Transmitter. However, LDS store found out the IP low a lot from factory setting. 120PSI. He then advise to increase to 145PSI.
The Legend LX should have a MP of 116 to 130 psi ( 8 to 9 bar). 120 psi was within factory specifications...145 psi is outside the factory specifications.
Reference: Aqua Lung Technical Maintenance Manual - Legend LUX, Legend LX, Legend First Stage