The low rumbling bass line produced by an idling train (or trains) up to a mile away. Permeates walls, double glazing and your brain. Equivilent to approximate 1000 boy racers idling in the car park.
Worker #1: That train bass is particularly intense today.
Worker #2: Indeed, been going all morning!
Worker #3: Dudes, I couldn't hear it before you said that... now its melting my brain.