What is the period of the noise that you are observing? If it's around
8 (+ or - 4) seconds, then you're probably seeing strong microseisms.
These can have very high amplitudes. They are surface waves that are
generated
in the ocean, especially during storms. There is a good web page
describing them here:
Of course there are other possibilities for noise, both cultural
and electronic. However these latter sources will not look like
microseisms.