I just finished a research article done by the National Aerospace Laboratory NLR on behalf of the Dutch government.

Title: Analysis research of low frequent sound in the village of Zuidhorn.

During the period from 21st of June until the 5th of July 2010 the research team had four acoustic array antenna's installed in 4 houses in Zuidhorn. A low frequent hum sound was measured between 10Hz and 200Hz (20 through 70 dB). The sound was mesaured sometimes in a single freq. and a combination of multiple freq. with a pulsating character. The level of the pulsating signals mostly changed with 10dB within 1/2 second through several seconds.

Trains and industry has been identified as some of the causes but the reason why the people complained so much has not been identified.

Source: [link to www.sodm.nl]

it's obviously written in the Dutch language.