A non-invasive diagnostic tool and method for detecting an obstruction in a coronary artery, the diagnostic tool including a signal processor adapted to receive signals corresponding to a heart beat from a plurality of acoustic sensors attached to the chest of a patient. The thermostatic mixing valve is configured to produce a mixed fluid of a particular temperature from a first fluid of a temperature higher than or equal to the particular temperature and a second fluid of a temperature lower than or equal to the particular temperature. The apparatus also can include an insert including a bore and an exterior surface.