378s and their pantographs

It is the unit passing over neutral sections and is a perfectly normal operation with AC units.

There are magnets positioned lineside as well as under the bogie/s. As the train passes over one set of magnets, the circuit breakers pop open stopping the draw of current over the neutral section. They close after passing over the next set of magnets. If these magnets didn't exist and the driver continues to attempt to draw power while on the neutral section, once back on the live wire, it could cause a severe overload on the train causing untold damage, as well as a brilliant flash, like drawing power over a 3rd rail gap, only more spectacular.

Seeing as the 378s and 377/5s are relatively new, the circuit breakers may sound rediculously loud because they are new, give them time to 'bed in' and they should quieten down a bit. They might also be bigger breakers because these new trains require more power to operate (air con, various other electrickery etc) so will sound louder too.