Fish sensitivity to pump vibrations?

It is often said on these NGs that the sound of the pump is small compared
to the sounds in a fishes natural environment. Being an advanced PADI scuba
diver I'm fortunate to have tested this theory out a few times and
personally I'm not convinced that this is true. Although sound underwater
travels a long way, it also decays quickly with distance. So, it is easy to
hear a boat from miles off, but it is relatively quiet until it gets close.
In fact, I'd descibe the underwater world as remarkable peaceful. I compare
this to the noise I heard when I put my ear to the fish tank and even though
the pump is quiet in the air, it sound like a pneumatic drill once my ear is
in contact with the glass and to me much louder than the fishes natural
environment - probably due to the proximity of the pump. In addition, the
sea noises are intermittent and random, unlike the constant buzz of the
pump. Now I don't want to get into a discussion about different tanks /
pumps / fish etc, I'm really thinking generically here...

1. Are the different sound volumes simply a trick of my brain (in the same
way that a room always look much brighter from the inside than the outside
because the brain "compensates")? Indeed, has anyone done any decibel
measurements to quantify this?
2. Is this constant droning harmful / torture to the fish? (I have seen it
written that "they look peaceful", but perhaps a visiting alien would
describe a man in a padded cell wearing a straitjacket as "looks peaceful"!)
I don't know how it would be measured, but again I'm interested in
scientific studies on this.

BTW, I did come across one site that had lots of info on the effects of
particular types of sounds (sonic booms, infrasound etc.) on fish, but it
was all "big scale" stuff and there was nothing on the effect on the
_constant_ buzz produced by a pump on a fish tank.

Cheers (and sorry for the long posting!)
Mark