After the launch of Starman, during the inaugural flight of SpaceX's heavy lift vehicle, the Falcon Heavy, PearsonArtPhoto set up a website to track various stats about Starman and his Roadster.
A recent addition to the site was the distance from Starman to the three main astronomical bodies of interest – The Sun, The Earth and Mars – and the speed of Starman (and his Roadster) relative to those three bodies.
In the plot of the relative speed for Starman, the relative speed of Starman to the Sun, although it has the general shape of a sine wave, seems to fluctuate quite often. Why is this?
Plot (click to embiggen):