Secondary frequency heard in background of many recordings in multichannel mode

[akester]

I have a Nooelec NESDR SMArt v5 configured in multichannel mode to watch two frequencies (119.3 and 121.9). SDR is set to a sample rate of 3.0, FFT set to 2048, gain set to 15. Using the Docker image with the attached configuration.

My issue is that a third frequency for the ATIS (120.15) is heard in the background of most of the recordings. Sometimes faint, sometimes quite audible (I assume that's related to the automated volume compensation and the strength of the intended channel).

I'm not able to replicate when listening via GQRX, nor does it seem to be an issue outside of multichannel mode. I'm not sure if this is a case where limiting the channel bandwidth as outlined in the Wiki would help or if there are other settings to adjust.

fft_size = 2048;
log_scan_activity = true;
stats_filepath = "/stats.txt";
multiple_demod_threads = true;

devices: (
{
        type = "rtlsdr";
        index = 0;
        gain = 5;
        sample_rate = 3.0;
        centerfreq = 120.6;

        channels: (
        {
                freq = 119.3;
                outputs: ( 
                {
                        type = "file";
                        filename_template = "TWR";
                        directory = "/recordings";
                        split_on_transmission = true;
                        include_freq = false;
                        dated_subdirectories = true;
                }
                );
        },
        {
                freq = 121.9;
                outputs: ( 
                {
                        type = "file";
                        filename_template = "GND";
                        directory = "/recordings";
                        split_on_transmission = true;
                        include_freq = false;
                        dated_subdirectories = true;
                }
                );
        }
        );
}
);

[akester]

Increasing the gain to over 20 seems to have fixed it, reading the manual again I saw it was quite low.