The DS-4000 may have detected and recorded ambient sound along with the intended audio signal.  For example, if the DS-4000 were placed on a flat, unpadded surface such as a glass or stainless steel table, it is possible that during recording the recorder would pick up the sound of its own vibration on that surface.

This behavior is more likely to occur when MIC SENSE is set to CONFERENCE because the microphone’s sensitivity is greater when recording in this mode. Try setting MIC SENSE to DICTATION.

If it is necessary to use CONFERENCE mode and impractical or undesirable to hold the DS-4000 for the entire recording, place the recorder on a padded surface, such as a book.  The padding will absorb the recorder’s vibration. 

You may wish to consider purchasing the optional ME-12 Noise Cancellation Microphone.  The ME-12 plugs directly into the DS-4000’s microphone jack for uni-directional recording in high-noise environments, such as lectures and seminars.  It will cut down on much of the background noise and result in a more accurate transcription.
Verification Source: Olympus Website