A howling or humming sound like a low, sustained “houuuuugh” – can be louder than the steam whistle.
This usually indicates a hole in the fire, where cold air is drawn through the grate, causing the boiler tubes to vibrate – absolutely undesirable, especially for rolled-in tubes!
However, this is not always the case. Especially not with the SRS marine-type boiler. Even when the firebed is laid perfectly, the boiler tends to howl under load due to a shallow firebed – a consequence of the grate’s relatively high position – which doesn’t give the air enough time to heat fully. At least, that’s the common explanation, which fits with the “fire hole” theory.
The simple fix is to introduce a little top air by slightly opening the firebox door. This reduces the vacuum in the firebox and thus the airflow through the embers. It doesn’t take much – just lift the lever off the closed position. The first notch would already be more than enough.
As a side note, top air can also help build pressure faster by improving the combustion of unburnt gases and producing additional heat.