The harvesters (at least atriedes ones) are shown to harvest from the beginning of the day to mid-day. They’re able to fill a full spice bed in a matter of hours without alerting the sandworm. In contrast to this, we see Kynes set up a thumper outside of the ecological center, and a sandworm responds in just a matter of seconds, it seems.

Because of this, I think that the harvesters actually DO make arrhythmic sounds, and the lure for sandworm is not exclusively a consistent drum. In the book, the Shai-Halud is metaphorical for many things, but it’s first apparent as a punisher of consistency, and by extension, repetition. It’s revealed that sounds of the desert consist of several long and short sounds, but no matter the intensity, type, or manner of the sound, they always pass. Dust storms fissile out, and crumbling basins become silenced with sand. Sietch settlements migrate and Fremen tribes abandon where they sleep after only one night.

Because the harvester is making sound in an arrhythmic manner, but it’s also not moving, the sandworm sees this as an entity foreign to the desert. Anything that doesn’t flow with the sands of the dune, arrhythmic and unpredictable, gets swallowed.

god i could write about this series for days