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There are diggerent levels of inverters. Some put out an actual sine wave like the waveform of the power in your home.
The next down will create a waveform close to the sinewave, but not quite. They use a little more battery power while in use, but are fine for motors and pumps. Most, in our hobby use this type.
The next down use a square wave that shouldn't be selected when buying one for rotating machinery.
The second and third types will produce lots of harmonics that can make the high frequency noise you hear. That noise simply gets stronger when you with a larger load.
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