Hi guys,
my car 1988 300e is doing the same. I noticed that when the fuel is running low, the humming is louder. But if I fill the tank up, the humming dissappears. I think it has something to do with the fuel pump intake or fuel backflow inside the tank. Because it didn't harm the performance, I don't do anything about that. I hope milhan's problem is the same. By the way, for your information. I changed one of the two fuel pumps before.