Feedback on A4 midi controlled by Digitakt

Hello everyone,
I am midi controlling channel 1-2-3-4 of my A4 Mk1 through Digitakt.
If I mute from Digitakt one of the A4 channels 1-4 I continue to hear a continuous feedback. It stops just if I stop the digitakt or if press stop on A4 several times.
Any suggestions?
Thank you