Aerco Benchmark 3

[hvacr]

Hello all I have an Aerco Benchmark 3 3,000,000 btu/hr input. I have set it up as per the manual at the different firing rates. manifold pressure constant at 1.8"wc. At 40-50 percent firing rate the thing is noisy. You can hear the pulsations in the first floor. Any Ideas or have you run into this before. any help would be appriciated.

[targetman]

Today I assisted our local Aerco Rep with the start-up of four Benchmark 3's. Two of them had what I would call a harmonic noise at 30%. The Rep had no explantion for this.