Cell Phones.

[dinamo2007]

This is kind of unrelated but at my school when we do shows in our black box we have to run a really long XLR for our headsets which pick up either a spanish or irish radio station depending on the day.

 

[mbenonis]

It sounds to me like you intercom is picking up some shortwave (could be mediumwave broadcast band [AKA AM], but more likely shortwave) radio. ClearCom runs are unbalanced, and thus are sensitive to noise on the line. The best solution is to check to make sure your XLR has good shielding on it, and if that doesn't help see if you can run it inside some conduit (grounded on one end to the ClearCom base station).