6 Z2 first stages and 12 Z2 second stages in rental and more to be added. Never had one squeal. My personal T2 never squeals and none of my customers has ever returned an Atomic due to squealing. I service many Atomics owned by divemasters, world class u/w photographers, instructors and other dive professionals throughout the year and never had anyone comment about squealing. They are excellent regulators, superbly engineered, made in America, and hold up as rentals better than any other regs we have tried as rentals which usually end up in the junk parts bin.
One follow up thought. We environmentally seal our first stages, not because of the cold obviously, but because it keeps fine particulate matter such as coral sand, etc that may be suspended in the water from entering the first stage. I don't know the conditions that preceded the reports of squealing but it may be due to contamination or improper rinsing of the first stage. I have photos of some disassembled Atomics that have been worked hard and literally put away wet that will scare you from lack of rinsing and care but no reports of squealing in over 7 years as a dealer/repair tech.