amplifier makes crunching sound

[noob with an RS 10+ year member]

hey i just installed my MTX 1501d. its wired in 4 ohms to power my idMAX. when i set the gain using a multimeter i noticed that with the gain all the way down there is no noise. but as soon as i turn the gain up around 20 volts it starts to make sort of like a crunching or spinning sound.. almost like the hard drive of a computer when its loading. the more i have the gain up the louder or higher the frequency of it. it isnt very loud but very audible when the speakers arent hooked up. is this normal??? other than that, seems to be powering my idMAX wonderfully so far. right now i have the gain set to 56 volts (800 watts) at max volume.

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