Abstract: Non-contact heat exchangers comprise a gas carrying duct which has fluid carrying tubes inserted therein so that hot gas may heat the tube fluid. This simple principle is extrapolated into a heat recovery steam generator for a combined cycle power plant where hot gas turbine exhaust gases are used to heat steam turbine feed water in order to produce steam. A noise/vibration problem may be generated at certain gas turbine loads which create stimulus frequencies at or near boiler tube response frequencies. The invention proposes an optimum arrangement of upstream baffles which suppress the stimulus frequency generated by the gas flowing through the duct.