The rings have sound effects that one would typically find in Japanese manga. For example, above there is a ring that reads "do" (ド), but could be used to mean "thud" (ドッ). "Dodododo" (ドドドド) might be closer to "rrrrumble", though.
As website Spoon Tamago points out, these kinds of sound effects are very common in Japanese manga.
They're less common on Japanese fingers, but retailer Mitsubai Tokyo has them, topping out at ¥23,100 for the larger characters (like "do", above) in silver.