Does this noise occur underwater or when you're breathing through the reg ashore? Lots of second stages make some really loud, obnoxious noises when you breathe them ashore, but make no noise underwater. I attribute that to general harmonics within the second stage and the lack of water to dempen them. That is no big deal.
I'm limiting my guesses to the second stage because you say the octo doesn't do this. Exhalation forces the diaphragm away from the demand lever and forces the exhaust valve to open. There is really not a lot of mechanical action going on at this stage of the breathing cycle. You say this happens at the end of exhale. When you're done exhaling and begin inhaling, the exhaust valve closes (by means of water pressure) and the diaphragm collapses and opens the demand lever, allowing air into the second stage. I'm not familiar with any Apeks regs built after about 2001, but if (and this is a WAG) the back side of the diaphragm has a metal contact plate, and if the orifice is tight enough against the poppet to cause some slack in the demand lever, then there could be some noise at the very beginning of an inhalation. It wouldn't be very loud, and I'm not sure I've ever heard one, but it's possible. Of course, if you don't have metal-to-metal contact between the diaphragm and the demand lever then it isn't gonna happen.
Take your reg to the shop and ask the techs there about it. I don't think it's a big deal, but peace of mind is valuable enough to warrant a trip to the shop. We get a lot of questions about the funny noise that Aeris AT-400 regs with the DVT make. We've traced it to air flow around the red stopper in the inlet when a breath is taken. It's nothing, but it can be disconcerting to the uninitiated.
By the way, I was totally unaware of all the little noises my own regulator made until I dove it after the first time I rebuilt it myself. I think just about all regs make little noises and they are probably as unique as the squeaks and rattles in your car.
Hope this helps.
