Hi,

Stick an ampere meter in between the negative cable and the negative post on the battery and find out how much the drain is drawing. This checks the whole car.
This will give you a clue to what item or items are doing the drain.

When you first replace the cable there will always be a tiny spark due to the dash clock or the computer charging up their capacitors. It should only be a few milliamperes at best fir a second or so.
You want to see only a steady reading in the neighborhood of .002 -.050 parts of one ampere max.
You have to watch which scales are being used or if on the auto seek scale, watch the decimal point movement.

If you can hear noise from the differential, that at the back of the car, while at the front of the car with the battery, you have a lot better set of ears than me!
If you are walking around, then yes, I believe it's a pump or something.
That should not happen unless a relay is stuck closed and the other poster it pushing you to the right place.
Unless the wiring is totally screwed up there is no way they should run with the switch off.

Get back to us with more clues, as there is a logical reason for this when it comes to electricity!

Phil