A compressor comprises an impeller wheel mounted within a housing defining an inlet and an outlet. The wheel has a plurality of vanes and is rotatable about an axis. The housing has an inner wall defining a surface located in close proximity to radially outer edges of the impeller vanes which sweep across said surface as the impeller wheel rotates about its axis. The inlet comprises a tubular wall extending away from the impeller wheel in an upstream direction. An enclosed chamber is defined between said inner wall and an outer wall and in communication with at least one opening in said in said inner wall. The outer wall is penetrated by at least one ventilation aperture that is designed to be connected via a conduit to a location upstream of the inlet and downstream of an air filter.