[Stephano, again I do not work for a competitor. I like the reg and would like to keep it once fixed. I am probably jinxed with Regs, I returned my ScubaPro 650X for good since it had a very strong resistance (continuous, not pinging) while inhaling, way worse than a rental reg.[/QUOTE]
It has been my experience that the really easy breathers tend to make some kind of noise at shallower depths. My X650 does the resonance thing, on some occasions, at the start of the dive but once I get down to depth, it stops. Is yours noisy through the entire dive/at all depths?
Having dove, in the past on a Mares high end breather, I experienced some of the same "resonance" at shallower depths. Either way, I can say that both regs perform extremely well! In short, I just think that its the nature of the beast but I fully understand if its something you find unacceptable. Especially given the kind of loot these things tend to cost!