The river Northcote runs along the Northcote road at a depth of about 70 ft used to be streams where the roads off it are through the fields feeding it 200 years ago. above it now at the south end is the flood sewer system which prevents flooding on the Northcote Road in heavy rain which was put in a few years ago, you can hear the water at various metal covers at the junction with roads running off broomwood road on the hill.
https://www.wandsworth.gov.uk/media/199 ... part_1.pdf
(what was done)
This system is probably where the noise is coming from via vibrations in old sewers maybe blockages water going over or around. Or even from old water mains in general.
This is what I think but not an expert in this field but have found it to be so in other parts of London. Years ago after putting in microphones to find the source of sound believed to be electrical but was found to be water-related.