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Do those mikes have an adjustable gain control ? Mine have, located in the battery compartment. I've had a slight scratching sound a time or two that was solved by just a slight reduction in the gain.
I agree with gduns, but there's an oustide possibility that there is an interference of sorts on one mike's frequency and not the other three.
If they're preset frequencies(channels) like mine are, you can't tell. If they're user set like some multiple units I'ver seen advertised, you might try swapping frequencies and see if the problem moves to the other mike.
Dunno, just brainstorming.
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