This article is from the Asthma FAQ, by Patricia Wrean and Marie Goldenberg email@example.com with numerous contributions by others.
Wheezing is the whistling or rattling sound that occurs when
air flows through obstructed airways. At the start of an
asthma attack, wheezing usually only occurs while exhaling, or
breathing out, but as the attack progresses, wheezing may
then be heard both while inhaling and exhaling. If after
the attack progresses further, the asthmatic then stops wheezing,
this may indicate that many bronchioles (small airways) have
become completely blocked, which is a very serious condition.