According to the BBC "An expert measured sound levels louder than a pneumatic drill and advised regular passengers to consider wearing ear protection." The expert in question measured noise levels on the Victoria Line and found that found that the noise peaked at 118dB, louder than a pneumatic drill. Apparently the average level was between 88 and 89dB.
Now I know that no one in their right mind is going to be on the tube for longer than eight hours but "Health and Safety rules state employers should provide ear protection for people exposed to average levels of 85dB over an eight-hour period."
Obviously it's a scaremongering article but there have been many times when I've noticed the noise is pretty loud - but then I live under the flight path at Heathrow so I get used to noise.
Professor Prasher the expert really got into his stride though:
"It's the equivalent of a big jack hammer going, or a jet engine taking off in the distance.
Although the journey times for most people are going to be short, this is a cumulative damage.
If you regularly commute and use that line, then you need to seriously think about some form of ear protection."