for the past six months i've had an annoying rattle coming from both fans on my 2015 macbook pro. it only happens occasionally at certain mid-level RPMs, never at low or high speeds. i had de-dusted the fans several times to no effect, and noted only a tiny amount of dust along the inner impeller blades
the usual answer in these cases is to replace both fans, which are not terribly expensive.
figured instead: why e-waste potentially good fans? i pulled them apart, and did a deep cleaning on the impeller blades using a toothbrush. it turns out that the tiny amount of dust buildup on the insides of the blades were enough to cause an imbalance on the rotor shaft, making the fan imperceptibly wobble at certain speeds. scraping off 10 years of cruddy dust was enough to rebalance the fan, and now both are operating silently.