I noticed after changing the GPU stock fans to the Noctuas, the Fan tachometer monitoring shows random spikes at some points in the graph. Like, its sitting at around 700 RPM, but suddenly there is a single data point at 1600-1650 RPM (close to the max speed of the fan), than back to 700 RPM. It seems to happen at around 2 times per minute, at random intervals, and the "% fan speed" graph does not show a corresponding spike. Oh, also, 50% fan speed in msi afterburner actually corresponds to the max speed of my fans.
Any idea why that might be?
I just bought a random sheet of black EVA, around 2 mm thick i guess. I've picked it because it's easy to work with, does not deteriorate and looks fine. But honestly any material that you can cut and is thick enough to fill the gap between the GPU and fans will do, even some cardboard perhaps. I do not use the fan grills. I'm using MSI Afterburner to control fan speeds. What GPU model are you using?
I've got an EVGA 1070 FTW so should be close to identical as yours physically.
No idea about the fans, sorry. I've mostly left this kind of thing at stock until now.