Lub, dup... lub, dup... lub, dup... The sounds of the heart can be heard by pressing your ear against someones chest or by using a stethoscope.  The first heart sound ("lub") is made by the vibrations of the closing mitral and tricuspid valves.  The second heart sound ("dup") is slightly higher pitched and is due to the vibrations caused by the closing aortic an pulmonary valves.  The "lub dup" sound represents one beat of the heart.  The first heart sound lasts about 0.15 seconds and the second lasts about 0.12 seconds.  The second heart sound is sometimes split into two separate parts.  This sounds more like... lub, did-up... lub, did-up... lub, did-up...This becomes more obvious during inspiration (breathing in).  Taking in a deep breath pulls extra blood into the right side of the heart which delays the closure of the pulmonary valve slightly.

 

Other heart sounds may also be heard in normal healthy children and adults.  A third heart sound is sometimes heard with a stethoscope after the second heart sound.  This is probably due to the rapid filling of the ventricles with blood.  Imagine the "thwop" sound of a cracked raw egg being dropped into a cup. 

 

A fourth heart sound is sometimes heard when the heart muscles walls are abnormally stiff or when pressure in the atria is high. 

Are all heart murmurs abnormal?

No.  Most children will have a heart murmur at some point or the other.  A heart murmur is simply a sound and does not necessarily mean there is a "hole in the heart" or a leaky heart valve.  Children have smaller and sharper angles in the heart and large blood vessels and this can lead to "normal" murmurs.  Imagine holding the middle of a garden hose with running water.  If you bend the hose slightly, you can hear the rushing water inside the hose.

Some heart murmurs are not normal.  Only your doctor can tell which sounds are not normal.  See article on heart murmurs.