If you have nearby strong FM broadcasts in your area you may need an FM trap in your antenna line.
Also, if you run your RF gain too high, you can overload the dongle.
On the 'Configure/ popup window, UNCHECK the boxes for RTL AGC and Tuner AGC.
Then adjust the RF Gain slider manually.
If you watch the the 'Spectrum Analyzer' part of the display, you 'may' see the FM signal go away at some point.
If the overload isn't too bad, you should still be able to hear other signals.
Rich
Edit: By the way, you may be stuck with that problem if the signal is too strong.
Edit again: The following screen capture shows two identical dongles at the very same RF gain level.
The one on the left has an FM Trap in the line.
The one on the right has no FM Trap.
Keep in mind that if the FM signal is strong enough, even an FM Trap won't eliminate it.