This is where I’d go as well since it’s being seen on multiple materials. I’d go through and remove everything and check for cracked wheels, missing teeth, areas rubbed down, anything that could be showing you that repeating pattern. Even an even but too thick layering of dust could cause a slow-down-speed-up which is what looks like is happening here. I would do each item one at a time (as opposed to removing everything and then having to figure out how to put it all together), and make sure both fan blades and intake/output openings are clean and clear. Get yourself as close to factory clean as you can.
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