I had the tank off my recently acquired '07 S40 to lube the speedo cable, clean wiring connectors, wipe the gunk off the frame, wiring & etc., and fix a newly present instrument cluster vibration/squeak.

I didn't even bother to take apart the spring / post mountings. That noise comes from the spring loaded mounting posts rubbing on the tangs on the tank right there at the rear of the speedo housing. I just put some  grease on a toothbrush and rubbed it thru the springs onto the posts, then put a drop of oil into the springs to keep things soft. Took 60 seconds. Noise is gone.

Heck, if the noise comes back it's nothing to re-lube and go on.

I pass this on in keeping with the Keep It Simple Stupid ethos of this fine list. Why do anything we don't need to do?