Humming noise under a Plymouth brezze

My girlfriends car just developed the most obnoixous noise ever. Under her car it is just hums like a alien is landing. yes your probably going to laugh but this noise doesnt go up or down it is just a solid hum. Its pretty loud and I really cant find the source because it is such a solid pitch.

Anyone have any ideas? it cant be a exhaust leak because the sound should increase with a rev . It almost sounds like a door ajar beeper noise under her car but its just a solid hum.

---------- Post added at 01:43 PM ---------- Previous post was at 01:29 PM ----------

just a rough guess from doing some research but i take it her fuel pump is going ( i can see this happening because she always drives around on a empty tank of gas). If the pump is going does it make any noises or just a constant hum/ whine noise?

Pump whining would be in the back of the car. Does it do it right at an idle? If it does rev the motor up and shut it off while it has some revs behind it. If it stops making the noise it’s most likely a pump or a clogged fuel filter making the pump work like a champ.

ya its in the back , yes does it at a idle and i will go and give it a few revs in a minute… i may change the filter hoping that may stop it for now because she wants to get a new car and doesnt want to dump money into it

just so you know the filter is kind of a pain in the butt on those. They want you to drop the tank to do it. It is possible to do it without dropping it though, you will simply need a long set of needle nose pliers to undo the tabs.

If you don’t want to drop the tank, i’d imagine you could take the rear seat out and get to it from the top. That’s atleast what I did when doing it on my sentra.

Are you getting any codes? Could be emissions equipment such as the vent solenoid or air injection pump. I can’t remember off the top of my head what emissions equipment is above the gas tank but on most Chrysler products one if not both of those are back there and they can both make a constant noise like you are describing.

no codes…may work on it today if I dont go jet sking