It’s a design flaw. Every single Steam Deck OLED will have it under the right circumstances.
With all due respect, people saying they aren’t having it clearly either:
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Are using headphones that are less sensitive
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Aren’t pushing the GPU enough for it to be a problem
Must admit I didn’t test with different brightness levels - you absolutely might be onto something here.
Will say with mine I don’t get any crackling or pops either, just a fairly persistent electrical noise.