angle to the axis of the combustion chamber 27 further restricting gas flow out of the combustion chamber unacceptable buzzing was found to occur tests were then conducted to determine the effect of varying degrees of restriction on the aperture of the exhaust stack discharge opening 56 and its relationship to the onset of buzzing in the burner the discharge opening 56 of the exhaust stack was blocked or restricted to varying degrees by placing plates with varying ratios of aperture area to plate area over the discharge opening 56 of the exhaust stack surprisingly it was found that sheet metal plates blocking approximately 50 of the discharge opening of the exhaust stack eliminated the buzzing a solution to the buzzing problem was therefore implemented by attaching a slotted sheet metal plate 60 to the exhaust opening 56 as shown in fig2 the plate 60 defines a grille with a total combined aperture of the openings 62 between the metal strips 64 of approximately 50 of the overall area of the plate enclosed by the four edges of the plate the plate 60 has an outer frame 66 which fits onto the edges of the discharge opening 56 of the exhaust stack 18 the plate 60 was found to suppress the buzzing in the burner 12 the use of such aperture restrictor plates 60 on the exhaust stack was found to be an effective solution to the buzzing problem in a number of different direct fired fluid burner heater units 10 of the type described above in one form of this invention.