Most of the time UFOs are silent; but a beeping sound can occur---this was mentioned in the classic u.s. case of Betty and Barney Hill. I heard the sound myself during the 1970s in a cabin in n. calif redwoods-------------it would occur only at night, was loud, hi pitched electronic beep, coming from all directions; it was a sound not of this world.----i never saw an object; only heard the sound.