This applies to the ISA 428
If you have a digital card in your unit then check both analogue and digital outputs for noise. If you can hear the noise on all the analogue outputs as well as the digital outputs check that the fuse holder on the rear of the unit (by the mains connector) is set to the correct supply voltage - the arrow head should point towards the correct value for your location. If this is set incorrectly e.g. 230 instead of 220, then it is possible that noise can be induced into the signal.
If the noise is still present then check to see if the noise level varies with pre amp gain. If not then it is likely that there is a ground loop. Check out this article on ground loops to learn more about them, how to determine if there is one and what to do about it. If the noise level varies with pre amp gain then it is being induced into the signal before the pre amp stage. Check cables and signal sources for faults and also ensure there are no sources of interference, e.g a transformer, which the cables are running past. If you do not manage to eliminate the noise then please contact us for further support.