Lucky Angel,
Experienced the same problem with my David Clarks, so sent them off to be tested. The microphone shielding was found to have broken down and having a new mic fitted has totally solved the problem. When there's two of you flying with DC's, there's barely a background hiss audible.
However, the company supplied Peltors seem to suffer the same interference, some worse than others, so I suspect its not an uncommon problem.
Perhaps someone can offer a more technically based answer?
JW