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most monitors have trim pots on the back to solve these problems.

But, usually there is no attenuation at maximum volume (in either channel) and the mismatch happens only at mid/lower levels. When you change the volume control the left/right balance changes and you can't fix that at the monitors.
It's a "shhhhhhh" noise that approximates the overallaverage frequency spectrum of normaleveryday sound. You can find a sample to download or there are plugins that can generate it.
Technically, it's white noise lowpass filtered at 3dB per octave. So, it's "less harsh" sounding than white noise.
Since it's noise it's highlyrandom and that makes "exact" meter readings a little tricky. But, it's better than a testtone because roomreflections can do "tricky things" with a single frequency that don't represent what's happening over the full frequency spectrum.
P.S.
Pink noise is also used for frequency response measurement. We are usually measuring (and equalizing) in octaves (or 1/3rd octaves, etc.). So for example, the octave from 200400Hz is twice as "wide" (200Hz) as the 100200Hz octave (100Hz). So with white noise (totally random) the wider 200400Hz range contains twice as much energy.
3dB is a power (energy) factor of 2. So if we use a 3dB per octave pink noise filter, both bands will read the same and our instruments will "show flat" frequency response.