Maybe a long shot and I am talking as a lowish hours NPPL but a very long time train driver. We encountered a problem with a new model of locomotive. The engine at full power had a lower cab noise than the next notch down which caused a howling noise in the cab which was unbearable for long periods. After extensive tests it was found that the actual dB level of the lesser power setting was lower than the full power setting, the excess noise was something called Harmonic Resonance. It would not show up on a decibel test of total sound and was of a frequency that the human ear was rather apt to pick up.
It was tracked down to an electrical relay that emitted a sound wave which at that power level caused the resonance within it's location and the resultant bloody noise. Relocation of the relay and having it somewhat insulated cured the problem.
Apologies for the cack handed explanation. Driver not fitter.