A fan rotor arrangement comprises a fan rotor (24) and a plurality of fan blades (26). Each fan blade (26) comprises a root portion (36) and an aerofoil portion (38). Each aerofoil portion (38) has a leading edge (44), a trailing edge (46) and a tip (48). Concave pressure surface (50) and convex suction surface (52) extend from the leading edge (44) to the trailing edge (46) of each aerofoil portion (38). An annular wall (54) surrounds the fan rotor (24) and fan blades (26) and an inner surface (56) of the annular wall (54) has a circumferentially extending groove (58). The circumferentially extending groove (58) is arranged axially, or chordally, between the leading edges (44) and the trailing edges (46) at the tips (48) of the aerofoil portions (38) of the fan blades (26). The circumferentially extending groove (58) extends axially by at least half a wavelength of an unsteady pressure wave to provide a geometrically tuned cavity and additionally pressure loss to suppress axially upstream propagating unsteady pressure waves. This reduces vibrations of the fan blade (26).