94 E320 Fuel pump problem.

[alexlepro]

I have a 94 E320 and when I first bought the car, the fuel pump was really noisy, making a loud hissing noise constantly when the car was on. I thought it was the fuel pump going bad so I replaced it with a new used one from a junk yard. The new one makes the same hissing sound, althought it is much quieter, it is still loud enought to hear inside the car when the windows are open and I would like to fix this. Is it normal for all of these pump to make this noise or should I look into a brand new one?