We use records from the Project IDA modified LaCoste gravimeters to investigate ground noise at frequencies from 1 to 10mHz. At most sites the level between 2 and 10mHz is nearly flat and close to 2¿10-18 m2s3. Much higher values which are observed at island and coastal stations are due to loading by waves trapped along the shore. Data from the superconducting gravimeter at Pi¿on Flat Geophysical Observatory show that the noise power increases as f-2.7 for frequencies between 1 and 0.001 mHz.