Is the radio connected to the original radio loom?
I have seen a few cases where there was a Parrot or equivalent before using a SOT plug (These have much thinner wire gauge size than OEM usually), those plugs were left plugged in to the radio and it causes background noise issues.
If that's the case, remove it and go back to the original plug.
If not try the latest firmware.
If still no luck, I would say send it back
In my experience, Grom Audio is a much better and reliable solution for the E39/E46/E53...