In this article, we treat the problem of disturbance rejection in event-based control. We consider the case of an LTI system under the influence of an output disturbance. We base our event-triggering conditions on the comparison between the actual system and a fictitious reference system with an ideal behavior. However, since we do not know the value of the disturbance, this comparison is made difficult. So, we propose a solution inspired from fault-diagnosis methods where we build an online estimator to estimate the value of the disturbance. We provide, then, this information to the reference system and proceed with comparing the states of the two systems.