Ok here's a weird one... I'm in the middle of changing the injectors on my car, and when I disconnected the return line from the rail, I heard this gradual release of air pressure, then a dull "thunk" from what sounded like the fuel tank, so I'm guessing something with the fuel pump. When I did this, I noticed it was the same dull "thunk" that I often hear about 15 seconds or so after I start the car cold (sometimes while backing up). It's got me curious, and I was just wondering if anyone knows.... What is that? lol

the fuel tank is under pressure. Disconnecting the return line relieves the pressure causing a thump noise. I'm sure you've heard the pressure escape when removing the gas cap when the tank is very low.

Yeah I figured on something like that...but why do I sometimes hear the same thump a little after I start the car then? No pressure is being released then...