My computer build now includes a piece of structural 8.5x11 printer paper.
I tracked the tapping noise to the case fan, poked at it a bit, and realized it got perfectly happy with just a bit of upward support. So, a bit of bridging between that and the top of the card cutouts just below, and I have a near-silent computer again! (Ignore the dust, that's the next task.) #lowtechtech
It's just dem dare city folks that obviously didn't stonk up on duck tape when had der chance.