As someone who did acoustical consulting, this is a favorite topic. In particular, doing battle with architects an interior designers who rely on HVAC noise and in some cases ambient noise generation to create some sense of privacy in open office environments. How anyone could believe that adding noise makes for lower stress work environments is completely beyond me. In one case the ambient level from HVAC was over 65 dBA throughout the facility, and it was billed as a 'quiet' work environment. It took over a year, dozens of employee complaints, stress related sick days, and even employees quitting before management relented and implemented my design. We reduced the ambient to below 50 dBA, and within a couple months productivity, employee satisfaction, and even attendance had improved measurably.
Also, regarding high efficiency systems, they benefit greatly from being properly gain staged so that you don't hear noise when the system is idling. Ideally, you want your preamp/line stage to clip within 3-6 db after the power amps. It's common to see preamps operating 10-12dB or even more gain than is necessary, and that robs the system of that much dynamic range and results like hissing horns at idle. Turn the amps input sensitivity down, and drive the preamp harder. That leaves the noise levels down.