*THUMB* is a noise made by three monsters: greater molochs, titans, and greater titans. All of these are bad news to any but powerful late game PCs, and even they have to be careful. Since titans and greater titans in particular can shoot extremely deadly eternium quarrels, low HP PCs would do well to avoid idling in that dungeon level if possible, or perhaps flee.
Since greater molochs move a lot more slowly than titans, one can distinguish them by measuring the frequency of *THUMB*s. Greater molochs are easy to flee from, but one can still get stuck between one of these and, for instance, a diamond golem. You can use a scroll of monster detection to determine whether you face molochs, titans or greater titans; they are a brown-colored &, light green and dark green H respectively.
The place an early to mid game PC is likely to encounter a *THUMB* noise is the Dwarven Halls. On a checklist of things that should prompt an early retreat and defaulting to the Animated Forest route, this is pretty high up. But if the player can find the stairs quickly and teleport to them the risk can be minimized.