DARPA’s Ultrasonic Cloaking Device
Unfortunately, it’s not quite up to 23rd Century Klingon standards:
This effort will use directed ultrasound technology to enable the capability to significantly reduce sound emissions from large scale tactical military hardware. Theory predicts that nonlinear effects of high-power acoustic radiation on the atmosphere can cause acoustic energy to dissipate rather than radiate. This theory has been confirmed in some limited experiments; this program will apply it to reducing acoustic emissions of U.S. equipment. Reduction in noise levels by at least 30dB would enable U.S. forces to effectively operate considerably closer to enemy forces without being detected aurally.
Not quite as cool as being totally invisible but this could certainly save a lot of lives.