Reversing sensors and metal bumpers?

Why do some reversing sensors not work in metal bumpers?

I ask this after having problems with a cheap set that i have just fitted. They do work most the time, but on occasion they emit a constant tone when reverse is selected. Particularily after its been raining. I did think it was the rain but after reading other stuff I see that some arnt reccomended for metal bumpers. So, does anyone know why? Just want to narrow down why I am having problems with them.

Ta Steve

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Steve
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