(Already awaits the "you shouldn't need them if you can drive" comments)
Lately I often get either a long single beep, or a long beep then a short gap, or long beeps at the same interval. This is with no obstructions behind me, then all of a sudden they'll work fine. If I put a finger on each sensor I can feel a light pulsing, they all respond the same way. If during the long beep then a short gap, covering a single sensor it changes to a constant beep as if the object was close.
Seems the sensor(s) respond all the same way, but is the issue a sensor of the control module? Mine are a factory fit & I'd like to keep it that way rather go aftermarket & alter the wiring of size of the holes in the bumper along with having them sprayed.