Hi needmynitrogenfix,
Most barrel poppet 2nd stages are pretty sensitive to orifice adjustments. A small turn of the orifice results in a noticeable movement of the lever.
I typically tune mine so that there is just the tiniest of leaks with the adjuster knob set to the easiest breathing position. During the dive, I set the adjuster to just stop the leak... this is usually just 1/8 clockwise turn of the adjuster knob or so. This increases the pressure on the spring, but does not effect the lever height. Turning the adjuster knob results in less "effect" compared to turning the orifice...
Since yours does not have an adjuster, you will need to ensure there is no leak at all; but this should only take a small adjustment from the "slight leak" position, maybe a 30 degree clockwise turn of the orifice, or approximately "12 o'clock to 1 o'clock". Turn orifice clockwise to "just" stop the leak, then just ever-so-slightly turn it clockwise a bit more to prevent it from leaking during the dive. It should still breath very easily.
You may need to re-adjust the orifice as the seat "takes a set" after a dive or two.
If I am understanding your problem:
1.) Your 2nd stage seemed too hard to breath, so you turned the orifice counter-clockwise with the inline adjuster.
2.) You dove the reg, it breathed well, but now had a slight free-flow.
3.) You used the inline adjuster again, this time turned the orifice clockwise.
4.) This stopped the freeflow, but caused a screeching noise during inhalation underwater.
You may have caused the lever to drop too far with your 2nd adjustment (turning the orifice clockwise drops the lever). This might now cause the diaphragm to vibrate or flutter against the lever.
I'm really just guessing, it could be something else entirely. But it seems to be related to changing the orifice position, which in turn effects lever height.
What concerns me with the additional history you provided is that the regulator became very difficult to breath a few months after the last service. That would cause me to completely disassemble the 2nd stage.
The normal failure mode or just general wear from use is for the soft seat to begin to wear, and a gradually worsening free-flow develops. But 3 months after a service is too soon unless you are diving an awful lot, and again I would usually expect a slight free-flow as the seat takes a "set", not hard breathing.
If the reg had been easy breathing, then became hard breathing in that short a period I would want to know why.
Since you are comfortable servicing it yourself, I would take it apart again.... start from scratch, be extra careful during assembly to make sure the lever "feet" are engaging the poppet correctly (shuttle valve is what Zeagle calls it I think), and re-tune it. This will allow you to inspect the diaphragm, lever, seat, spring, orifice, etc.
Best wishes.
