Weird signals in external loopback

[zsoerenm]

I've configured the bladeRF micro 2.0 in the following setup

                             ┌---- RX1
TX1 --- 30 dB Attenuator ----|
                             └---- RX2

The bladeRF Sink is fed with a complex continuous wave signal at frequency of 600 kHz with a sampling frequency of 6 MHz.
The bloadRF Source is configured to receive this signal. I've attached the GNU Radio file here.

Here is the spectrogram of the received signal.

The largest peak is the wanted signal at 0.6 MHz. The other ongoing peaks come from IQ imbalance, I guess. However, there are some signals that appear and disappear from time to time. Do you know where this is coming from?