Savart wheel

Savart wheel is 17th century mechanical sound generator.

You put a gear into dremel, turn it on and carefully touch the wheel with piece of paper/cardboard. It will make somewhat clear pitched sound. The faster it spins the higher the pitch is.

Or use magnets instead of gear tooth and glue a small magnet to a piece of paper. It is safer and produces clearer sound (see screenshots of "Spectroid" Android app).

Obviously do not forget to wear safety googles and take every precaution while working with power tools.

I used 10 mm diameter 1.6 mm thick magnets (1x4 for one design, 4x4 for another), 13 mm hole for my ~12.5 mm sanding mandrel. If you need to adjust any parameters then take a look at OpenScad files.