My 380 SL did the samething. I went under the car with it running and used a stethascope (sp) to listen and the noise was definitely the fuel pump. I was told by many that it might run indefinitely like that but also it might just quit and leave me stranded. The common idea from most was that if it makes that hum something is going wrong. I decided to go ahead and change mine which is not very difficult. Humming noise went away and all has been fine since.