EMU sounds like steam locomotive

[ccityplanner12]

Every now & then here in South London, one encounters an EMU which makes a rhythmic sound every couple of feet. At speed this sounds remarkably like a steam locomotive. My first thought as to what causes this is unevenly-worn tyres. Most of my encounters with trains that do this have been in London, although I have come across this phenomenon elsewhere, including in other countries.

 

[D365]

Wheel flats? The Class 455s are very prone to this.