Abnormal breath sounds: Crackles, Wheeze, Rhonchi and Stridor

Crackles (Also Known as Rales)
What do crackles sound like?
Roll your hair between your fingers next to your ear.. That's what fine crackles sound like! (Or the sound of salt heated on a frying pan, if you've ever tried cooking :P)
Coarse crackles sound like ripping open Velcro.

When are crackles heard?
Crackles are much more common during the inspiratory than the expiratory phase of breathing, but they may be heard during the expiratory phase.

Mechanism of crackles:
Crackles are the sounds you will hear in a lung field that has fluid in the small airways or if atelectasis is present.

Causes of crackles:
Crackles is often a sign of adult respiratory distress syndrome, early congestive heart failure, asthma, and pulmonary edema.

Rhonchi
What do Rhonchi sound like?
Try making a snoring sound or try to make a sound as if you're gargling your mouth. That's what Rhonchi sounds like!

When are Rhonchi heard?
Rhonchi are more prominent on exhalation.

Mechanism of Rhonchi:
Secretions in large airways, as occurs with bronchitis, may produce these sounds; they may clear somewhat with coughing.

Causes of Rhonchi:
Pneumonia, chronic bronchitis, and cystic fibrosis are patient populations that commonly present with rhonchi.

Wheezes
What do wheezes sound like?
Try whistling slowly while exhaling.. That's what wheezes sounds like! Wheezes are continuous musical tones. 

When do wheezes occur?
Wheezes are most commonly heard at end inspiration or early expiration.

Mechanism of wheeze:
As the airway lumen becomes smaller, the air flow velocity increases resulting in harmonic vibration of the airway wall and thus the musical tonal quality. They result as a collapsed airway lumen gradually opens during inspiration or gradually closes during expiration. 

Causes of wheezes:
Heard when listening to an asthmatic patient. Sibilant wheezes are caused by asthma, congestive heart failure, chronic bronchitis, and COPD too.

PS: Wheeze is also known as sibilant rhonchi.
Rhonchi is also known as sonorous wheeze.

Stridor
What does stridor sound like?
Stridor are intense, high-pitched, continuous monophonic wheezes (musical sound) heard loudest over extrathoracic airways.  Similar to a creaking or a grating sound.

When is stridor heard?
They tend to be accentuated during inspiration when extrathoracic airways collapse due to lower internal lumen pressure.

Mechanism of stridor:
It usually caused by a foreign body obstruction of the larger airways, such as the trachea or a main bronchus.

Causes of stridor:
It is also the most common type of breath sound heard in children with croup (Laryngotracheobronchitis caused by parainfluenza) and a foreign body airway obstruction.
Also heard in Bacterial tracheitis, Diphtheria, Epiglottitis (H. Influenza) and Broncholitis (RSV).

Stertor
What is Stertor?
It is like stridor but it's more harsh, less musical and low pitched.

Mechanism of stertor: Respiratory sound that originates from pharynx, basically, anywhere  above larynx.

Causes of stertor: Seen in partial obstruction  of airway above larynx characterised by heavy snoring and gasping like in obstructive sleep apnea.

Thanks Nab, for explaining stertor to me!