If the signal is provided by a GSM repeater, the problem described by you can be due to repeater auto-oscillation (part of the signal comming out is “feeding” the input of the repeater) !!!
The problem can be corrected if the following condition is satisfied:
Antennas separation > Repeater’s Gain
So, either you increase the separation between the pick-up and donor antennas or reduce the repeater gain.
Most of the modern repeaters may have a AGC circuit (Automatic Gain Control).