ScottyBoy's right, the only thing under there is the fuel pump. There could, but i very much doubt be a small blockage. Either that of the pump could be on its way out.
Might sound daft but if you can take it out, give it a few smacks while the engine is running (aka the pump is running) it might stop making the noise. Depends how loud/bearable it is!
If you do that, be careful not to knock off any pipes or you will have petrol everywhere!!!
If you were running low on fuel and air was being pumped through, you filled up again and its back on fuel it shouldnt really make a difference, now anyway. It would possibly make a noise if it was trying to pump fuel through but only found air in your tank!
You should hear the fuel pump prime up when you first turn the key in the ignition.