Rhaski t1_j5njwbu wrote

No. It's resonance. As it spins, it increases and decreases speed in a periodic fashion. This frequency is a harmonic just like when you flick a wine glass and it rings for a while, but instead of being an inward/outward flexing motion, it's an angular oscillation. This happens in the rotating assembly of some engine configurations too (such as V6 engines), so they have to have a harmonic balancer to smooth out this angular oscillation or it makes everything vibrate horrible when the RPMs match the harmonic frequency of the crankshaft and it can break. It's also observed in shaft couplings like U-joints. This is why the Constant Velocity (CV) shaft was invented, because traditional U-joints operating on an angle (such as in driving a front wheel whilst turning) caused fierce angular vibration to travel down the driveshaft and at the right frequency, it could resonate and lead to failure of the driveshaft or other components. We are seeing this on a much larger system and, consequently, we see a much lower harmonic frequency. In the order of cycles per century rather than cycles per second