GSM cell phone communication. Try routing the cable where no audience member sits or sets down their bag (and crew members carrying live GSM phones are to be treated accordingly, and switching the ring tone off is not enough). Longer connections should always be done with a balanced connection. If that's not the case, using an appropriate mic preamp with balanced output and placing it close to the mic might be an option. And don't use high impedance (or "instrument") inputs at either mic or line level. When you are already using a balanced connection, a better cable (additional shielding, more robust mechanics, more dependable inner geometry, refusing to bend sharply) may decrease the susceptibility to such noise.
Now you state "in certain places on campus" and GSM phones are not usually installed at fixed points. That makes it somewhat likely that you are carrying the culprit around in your team, and sometimes a person or bag with a cell phone on it gets placed close to the susceptible connection whenever you are at such a "certain place". If you suspect someone specific, send him a text message (SMS) or give him a ring. That's pretty certain to trigger the interference, immediately before the phone announces the reception of the message/call.