Could it have possibly been a Sherwood? Brut? Its a pretty tough reg and used allot for high volume rentals. The ONLY reason why this strikes a memory with me is because I just went to a Sherwood/Genesis repair seminar and the instr. covered this.
Believe it or not, its actually the spring inside the 1st stage. If its flipped over, then the noise goes away. Its the ONLY one that I know of where spring orientation makes a difference.
During regular maint/rebuild-it must have been inadvertanly flipped. I NEVER would have figured it out on my own, had the instr not covered it. On a side note, it will ONLY do it at depth(below10') so bench testing will not duplicate the problem.
He referred to it as a "long honking" sound.
Hope this helps.....