This paper first gives a brief review on brake squeal mechanisms and then studies in-plane modes/friction process and their contribution to disc brake squeal. Pulsed laser electronic speckle pattern interferometry was used to acquire the operational deflection shape (ODS) of a disc brake when it was squealing. Laser vibrometry was used to obtain mode shapes of brake discs/rotors including both the out-of-plane (transverse) modes and in-plane (radial or tangential) modes. The rubbing friction process with a non-rotation rotor under a free-free boundary condition was used to simulate friction-induced vibration. The coupling between in-plane modes and out-of-plane modes/vibration is believed to be the key to produce squeal. The in-plane modes tend to control the squeal frequency, and the out-of-plane modes/vibration are efficient to generate noise. Many case studies have shown that high frequency disc brake squeal occurs at one or some of its rotor in-plane resonant frequencies. Test results and analysis conclusion are provided.