Concert Unit Interference on A8s

I have noticed on more than A8 that the concert stereo unit faintly picks up the noise of the engine as the revs rise and fall. I've asked an Audi dealer about this and searched the Internet but can't find anything. Has
anyone else noticed this? Is there a cure or just something that has to be lived with?
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Could be a poor quality earth.
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Thanks for the response. I've further discovered that there is severe interference on my S8 when the CD changer is operating. This noise does not sound like the engine as it does not rise and fall with the revs, but it does disappear when the engine is switched off.
Can anyone advise me where I should look for a bad earth?

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Oh yes, our poor earth is certainly in a bad shape these days..................
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