Question 1st stage whistling below 13m

[Bloody Marvelous]

One of my first stages starts whistling when I go below 13m.
I tried breathing slow, that reduces the noise level, but prolongs the whistling.
I tried inhaling sharply, but that just makes it really loud.
It also whistles when I use the inflator.

As soon as I ascend above 13m, the regulator is dead quiet again. Drop below 13m and it starts whistling again.
I just certified for Sidemount, but I can't dive with a whistling 1st stage. It's annoying as all heck and it bothers the other divers.

Is there anything I can do in the field to fix the issue, outside of getting it serviced?
(Not that I think it matters, but the regulator in question is an Apeks MTX).

 

[Tanks A Lot]

Whistling on a first stage usually comes from two sources:

• On first stages of all types, from harmonic resonance between the bias spring and other first stage components. To get rid of this harmonic resonance, the following are suggested by almost all manufacturers, in this order: Flip the spring around; Rotate the piston; Exchange the spring; Exchange the piston.
• On Flow-through piston regulators, air flowing past the seat into the piston bore. Historically, manufacturers have used a cone shaped seat to mitigate this.

As your MTX is a diahpragm first stage, the obvious culprit will be the bias spring and you can disregard anything concerning pistons from the above two bullet points.

From the attached schematics, which are for the MTX-R not MTX, look for spring #AP1475.

1. Unscrew the Environmental End Cap #AP2010.
2. Remove the Pressure Transmitter #AP1483. It usually falls out when turning the first stage upside down.
3. Unscrew the Spring Adjuster #AP1474 with an allen key.
4. Flip the Spring #AP1475 on it's head.
5. Assembling is the simple reverse of the above.

However you do need a full cylinder and intermediate pressure gauge, as the spring adjuster sets intermediate pressure. It is crucial that the spring adjuster is not tightened too much or too little. Without a intermediate pressure gauge this is impossible to set right. (Page #11 of the attached manual)
The environmental cap must be installed while the first stage is pressurized. (Page #12 of the attached manual)

While the above is really not complicated, a couple of things are needed that probably fall into the specialized tools category, like intermediate pressure gauge.

On very few balanced diaphragm first stages, I have seen whistling not from the main bias spring, but the spring touching the HP valve. This would correspond to #AP1415 in this particular schematic. This has been very rare in my experience and usually was a very slight, subtle whistle.
The main bias spring is the cause in most cases.

 

[Bloody Marvelous]

Thanks for the detailed response. I'm on my last diving day tomorrow, so I'll just get it serviced at my LDS.
This is not something I'm qualified to do myself, and the dive center just closed .