Query about fuel pump noise!

Hi

Looking through the old threads i know fuel pumps/pump noise has been
done several times, I'd just like a bit of extra info please! I own a
1996 A4 1.8 petrol, over the last week or two there has been a low
pitched whine coming from the back of the car, same area as the fuel
tank but it does not start immediatly, it usually kicks in after about
3 miles. What i have not seen on any other of the threads is the fact
that when i brake the pitch of the whine drops but the voulme stays the
same (this is the same even when stationary). whilst trying to trace
the source of the noise earlier today, i heard quite a loud hissing
coming from the filler cap whilst the engin was running (yet there was
no smell of petrol so i dont think vapour is actually escaping), as
soon as the car was switced off this stopped however, over the last few
days i have been hearing a gurgling sound from the tank when i have
stopped and got out of the car, this lasts for several seconds then
stops. finally the petrol guage has started playing up also. if i fill
the tank right up the guage only shows 2 thirds full and then still
with a third to half of a tank left its showing empty with the petrol
warning light on! anyone got any ideas if this matches the what seems
to be quite common problem of fuel pump noise!?!?!