Bus Termination Filters, aka. snubbers, are not needed if you use speaker wire for your main bus. The reason for the noise [we think] is that the filters draw a large current spike every time the DCC waveform switches polarity. Our guess is that the current spikes are causing magnetostriction (mechanical change due to a magnetic field) in the current transformer (T1), and because they are short pulses, they can be heard sometimes as a ringing. The best answer is to wire the layout so that the snubbers (bus termination filters) are not needed. We recommend using speaker wire as the primary DCC Power transmission bus. It’s cheap and available everywhere.