[LX5liter]

It is possible that your fuel pump relay sticking on and keeping your pump going. I believe on the 91's the relay was under the drivers seat. Check to see if power is still going to the pump after you turn the key off. If the relay is O.K. it may be the check valve on the stock fuel pump is leaking slightly and causing the check ball to vibrate slightly as the pressure leaks down. You can check that with a fuel pressure guage to see if the pressure is dropping while you hear the buzzing noise.

I hope this helps.