In 1994, researchers at the University of New Mexico sent out a survey to 8,000 residents near Taos, N.M., to ask them about a mysterious low-frequency hum that had been bothering people there for at least three years.

Responses came in from 1,440 people, and 161 reported hearing the hum. Most of them said it sounded like an idling diesel engine, exactly the sound some people in Snohomish County have been experiencing in recent years.

Some of the people in Taos who said they had not heard the noise complained that the survey was a waste of time and blamed “newcomers, new-agers or pot-smoking hippies” for having overactive imaginations.

One of the researchers, Jim Kelly, an ear specialist at the time with the university, defended the hum hearers.

“We’ve gotten letters from reasonable people living all over the country,” Kelly said.

Of the 161 hum hearers:

* 73 percent were between the ages of 30 and 59.

* The genders split roughly evenly.

* 67 percent had heard the hum for one or more years.

* 75 percent heard the hum daily, every few days or once a week.

* 5 percent heard the hum continuously.

* 17 percent had heard the hum at one time.

* 62 percent heard the hum late at night or early in the morning.

The survey and subsequent audio tests by university engineer Joe Mullins led researchers to believe that the hum was not an actual sound.

They hypothesized that a small percentage of the population might be suffering from an unknown low-frequency hearing disorder that makes the brain think it’s hearing something. But they never got funding to pursue that theory.