Fuel rail pressure sensors(FRPS) are located on the front driver's side of the rail. I wish you could isolate the noise somehow to find where it is coming from. Also, when was the last time the fuel filter was replaced? The small in-line expansion regulator/check valve(silver thing on the fuel line) might be partially clogged......
I did replaced the fuel filter the first day I purchase the used car. As for isolating the noise, I wish it was repeatable. The funny thing is, I had the wife come out of the house to give me a helping hand more then once and the noise never came back while she was behind the wheel, go figure. She believes I'm having a senior moments.
I hate to admit this, this is the very first Ford I ever own. As a former Chevy guy... Go ahead I'll wait... I had a K2500 TBI that primed the fuel system for a split second when the key is first turned to ON position. I'm going with the theory Ford does the same, correct me if I'm wrong.
You do bring up a good source were the noise could be emitting from, (regulator/check valve(silver thing on the fuel line).
I do remember seeing this part, I thought this was for the AC, located on the passenger side near the firewall and very near were the IAC I replaced thinking this was the source of the noise.