This was also a good excuse to use my hydrophone, bought from Jez Riley French (http://hydrophones.blogspot.com/) as a contact microphone. What I got are some dark, industrial sounds, reminiscent of the cold concrete onto which the mic was attached. Interestingly enough though, whilst I was right underneath the motorway the sounds heard were predominately from vehicles passing beside me, the tunnel acting as a reverberating force whilst traffic thundered above. Here is that very recording...
Below the M11 by miles out in the field