Mechanical Chime Rings Continuously

Continuous ringing is a common symptom of a failing transformer.
If the transformer has been installed in the home for 15+ years, it may be time for it to be replaced.
The voltage on a 16 VAC transformer needs to be a minimum of 19 VAC.
The voltage on a 24 VAC transformer needs to be a minimum of 28 VAC.

A 10 VAC transformer needs to be upgraded to either a 16 or 24 VAC transformer.
If the chime is buzzing, it may require a 10-12 ohm/0.5-3 watt resistor between the Front and Trans on the chime. If it is ringing non stop a resistor/ diode will not fix the nonstop ringing.

Ensure that there are no diodes or resistors on the line. (As previously mentioned a resistor will only fix humming)
If the customer is using a mechanical chime please be sure they have not enabled the digital chime setting, this can cause issues.
Ensure that all wires are tucked in accordingly and not touching.