I have a couple of KX-99 AM two-way radios. On of them (Bendix) receives well, but the other one (Bendix-King) has a strange tone and doesn't seem to pick up an

I have a couple of KX-99 AM two-way radios. On of them (Bendix) receives well, but the other one (Bendix-King) has a strange tone and doesn't seem to pick up any signal. The tone is dependant on the frequency it's monitoring. Maybe this is a by-product of a superheterodyne circuit only present in the newer models? I wonder if any radio-savvy reader will be able to point me in the right direction here :)