Intrusion detection systems create large amounts of alerts. Significant part of these alerts can be seen as background noise of an operational information system, and its quantity typically overwhelms the user. In this paper we have three points to make. First, we present our findings regarding the causes of this noise. Second, we provide some reasoning why one would like to keep an eye on the noise despite the large number of alerts. Finally, one approach for monitoring the noise with reasonable user load is proposed. The approach is based on modeling regularities in alert flows with classical time series methods. We present experimentations and results obtained using real world data. Categories and Subject Descriptors C.2.3 [Computer - Communication Networks]: Network Operations--Network monitoring; C.2.0 [Computer Communication Networks]: General--Security and protection General Terms Security, Experimentation