yup. thats whats making noise, and it is going to fail. ask me how i know. . .
and you're right, its under the seat and there is only that one. lift up the cushion and you should see an access port to it.
i have changed the pump on my last 91 E30 325i and a ford tempo. the ford one was cheap, but it was a PITA to replace. had to drop the tank.
the BMW one might be expensive. but its so easy to replace. its a trade off, and ill take the BMW route anyday.