Noisy fan. CPU temperature starting to rise (measured with Hardware Monitor Program), so the fan was clearly about to fail.
Lots of screws! Don't mix them up. While a 00 Philips is the only tool required, I found a pair of strong reading glasses and tweezers very helpful. Actually, I stacked TWO pairs of reading glasses for extra magnification; this is what come of eyes of a "certain age".
Be very careful of the screw at the right rear of the fan, the one that is closest to the screw on the Macbook power button side. Be very gentle with those closely routed cables, because they don't have much slack. Don't put in the fan and then the fan case screws, instead transfer the screws from the old fan and move the fan case in with it holding the screws.
Some, but not all(!) of the screws attach to a magnetic screwdriver. I think some are stainless and hence non-magnetic.