We usually think of a heart attack being preceded by obvious symptoms such as chest pain, but a new study finds that almost half of them are “silent,” and can go unnoticed.

Researchers at the Wake Forest School of Medicine in Winston-Salem, N.C., followed nearly 9,500 men and women for nine years.

During that time, 3.3 percent of participants suffered a silent heart attack while slightly 4.1 percent suffered one that was preceded by symptoms, according to a report in Circulation.

A study last year found that nearly 80 percent of heart attacks are not diagnosed — and therefore not treated — because they are silent heart attacks.

But even most silent heart attacks are not completely without symptoms. In fact, they are preceded by unconventional symptoms like indigestion or muscle pain.

People with diabetes need to be particularly aware of the possibility of a silent heart attack because the disease causes nerve damage that can mask obvious symptoms.