I reseated the control unit (in the boot on the right hand side behind the carpet) and also cleaned the contacts of each sensor. That lasted a year before I finally replaced the faulty sensor.
It's quite straight forward taking the rear bumper off.
Someone has unplugged the speaker , it's in the drivers footwell under the steering column
Bmw-E34-E36-E38-E46-E53-E83-Gong.jpg
You'll need to drop the plastic panel down
Would be my first thought as well. Although, I had the same issue, even though the speaker was plugged in. Sensors were fine, but turned out the wiring loom had an intermittent short to ground. Replaced that and have had 0 issues since.