|I'm assuming you have separate amplified speakers. Sometimes a nearby strong transmission can be picked up by the speaker leads and fed back into the speaker amplifier where it is detected by some non-linearity.|
Try turning up the software volume and turning down any control on the speakers themselves.
Also pull the speaker plugs in and out a few times and rotate them. This will ensure that oxide is not causing a high impedance.
Another trick is to wrap each speaker lead back and forth for a few inches near the speaker and tape it together (minimising the slack). This can reduce the pickup and also have a cancelling effect on the unwanted signal.
Note that if the signal is not being picked up by the speaker leads it might prove better to reverse the process given earlier, ie turn the software volume down and the speaker volume up.
Ms Hulot's boat