240 1991 humming from fuel pump or sending unit?

Hello,
Yesterday I noticed that there is a "new" humming/buzzing sound
coming from behind the rear passenger seat area. I assume that
it may either be the gas pump or sending unit? Is it getting ready to
go out? Is the filter getting old/dirty and causing one of the above
to
over work. Are either of these fairly easy to replace? How would I
know which one to replace, should I do it soon or wait until the unit
stops working or the buzzing drives me bats.

 

It's the fuel pump, there's nothing to hum in the sending unit. Does the
humming go away when the tank is full? Usually what happens is the
pre-pump in the tank or the hose connecting it fails and that causes
cavitation in the main high pressure pump under the car which is the
humming you hear. If this is not fixed, the expensive main pump will
eventually fail. None of this is terribly hard to work on, if you can
change a water pump you can change a fuel pump. Do it now, it would be
silly to wait until it leaves you stranded, when the main pump fails
you're suddenly dead in the water.

 

Thank you for the reply....I have a couple of questions..please work
with me here.
The sound just started a couple of days ago, so I don't know if it
hums on a full tank.
I don't want to fill it up if I need to open it up to replace the pre-
pump.
So as I understand there are two pumps, the main one is on the outside
of the fuel
tank, and a pre-pump inside the tank? When I replace the pre-pump I
understand
that I first off remove the fuel tank and then open it up and replace
the pre-pump.
How do I drain the fuel to make it easier.
Thanks again for your help.
Joseph Segura