Sound Golem.
Using particularly obscure and tricky magic, the crafter has to collect and contain noises and sounds for the construction of this golem, then bind them together magically into a single being. Having no physical form, a sound golem can transmit itself through any substance capable of conducting sound waves.
A Sound Golem constantly emits a barrage of noises - those sounds from which it was crafted, though normally it keeps these down to a respectable volume.
It attacks by occupying the same space as the creature it is attacking. It can deal lethal or nonlethal sonic damage in such a fashion - when dealing lethal damage its volume is much louder, rising to a deafening cacaphony. Typically it wanders around dealing nonlethal damage so as not to destroy the walls and objects it is passing through. Switching from nonlethal to lethal damage or back again is a full round action.
If a Sound Golem attempts to move through a physical barrior while in "loud, lethal damage-dealing" mode, it is limited to moving 5' per round through the substance, dealing its full damage to the substance per round.
Though it has no form, a Sound Golem still takes up a physical space, roughly equivelent to a four foot wide sphere. It possesses DR 10/magic, in addition to the regular protections of an incorporeal creature.