Given, that the Beolab 3500 is connected via Masterlink and it only happens with DAB means very unlikely that the problem is associated with the Beolab 3500 or its connection to masterlink. Rather it points to an issue in the Beocentre 6.
Did you purchase the Beocentre 6 with the DAB or was it subsequently fitted by your dealer ?
When you switch on the Beocentre 6 by pressing Radio from standby, doe sthe Beocentre 6 give any noise in the speaker in the same way as the Beolab 3500 ?