We combined noninvasive ultrasonic and optical measurements to estimate the mechanical properties of forming colloidal films. Light reflection measurements determined the stage of drying and film structure. A concurrent ultrasound measurement quantified the film stiffness. The main finding was that compressing capillary forces induced a temporal peak in film stiffness when air began to enter the pores in the film. We believe that empirically observing such a stiffening event has not been reported before. This finding advances the understanding of the physics of consolidating suspensions.