Yesterday I changed the rear diff fluid & gasket (you should have seen that sludge).

Anyway, I take the car out for a cruise to check for leaks (none!) and come back and park it in the garage. I shut it down and notice a faint humming/buzzing sound coming from the rear of the car. I put my ear on the quarterpanel just above the fuel door and can hear it coming from inside.

This is my 2nd f-bod and have never heard anything like that before so I pulled the PCM fuse to see if that would kill it and it did.

I am thinking it was either the fuel pump was still engaged or the evap canister was purging. What I do not know is why after the engine was shut off? I can hear the fuel pump when the car is on (it's pretty loud) and this was not the same sound. I thought the Evap canister only purged when the car was running so it could burn the fuel vaoprs?

 

Do you think the previous owner put an aftermarket fuel pump on there?

 

Totally possible. I can totally hear the pump running when the car is on. The humm I heard yesterday was different though, not as loud as the pump normally is.

 

It's sounds like your fuel pump. Mine does that when I drive it for an extended period of time on a really hot day on a bumper-to-bumper traffic, and I have less than half a tank of gas. The fuel gases gets pressurised and makes the pump hums. What I've done is to carefully open up the gas tank cap to relieve the pressure, if I'm away from a gas station.
If it's been doing it under normal driving conditions, your fuel pump is most likelyto go bad. The humming sounds that I have experienced is like a horn from a train kind of fading.
If it's a hissing noise, it could very well be you charcoal canister being full, although it should trigger an SES code. Soemetimes the charcoal canister can emit a bad odors like rotten eggs.
-Goose

 

WHen i first got my car, I heard the same thing..... I have had the car for almost 2 years and no problems with fuel pump yet.... What i did was slowly open the gas tank cap... and the sound was gone..

 

Weird...well thanks for the input guys. I hate the thought of my fuel pump being on the way out, but it has generally worked fine for me up to now. I will have to keep an ear out for this noise again.