All system tones are generated by the CPU. If there are some tones missing or garbled, I suggest you first measure your power supply voltages 5VDC, 27VDC in both PIMS, then try a system reset. If system tones remain corrupt, then your CPU may be faulty.
Is that tone the same using the handset? All multilines? All CO lines?
It sounded like an over modulated tone. Has you Telco provider changed anything lately?
Are you receiving proper loop current and voltage from your provider? -48V, 26-50 mA