looking for help in identifying the source of a humming noise.
It sounds like a pump running in the rear of the car, underneath.(1998 accord coupe). this is with car parked and ignition off. it seems to cycle on every 60-90 seconds and last about 10-15 seconds................

 

My guess is the EVAP system letting off pressure from the gas tank. That usually happens silently, but you're not the first person to notice the sound.

EVAP canister is roughly underneath the left-rear seat. Pressure should be released out from a tube right near the rocker panel, so very close to the front of the wheel-arch.

It's normally a very quiet sound like air coming out of a small tube/filter. I've never heard any more followup information from others, when it's louder, so I don't know if that's an early warning about anything? Is something gonna fail within a year? Within a decade?

btw, WELCOME new member!

 

you could also be hearing the fuel pump kick on you will only hear it for a few seconds

 

Fuel pump shouldn't run when the car's parked with the ignition turned off.
You can hear the fuel pump (for comparison) by turning the key to ON (but not immediately to START). Pump will run for 2 seconds then turn off.

 

You are correct Jim, after posting this I went out and listened more , then decided to unscrew the gas cap.............No more noise. thanks!!!

 

I agree. I stood, hand on gas cap and waited. Vent or pump noise could be stopped each time by venting the gas cap. Years of concern now history. Thanks, All.