About 1 hour after parking and shutting down the car in the garage, I was out there again and heard a humming sound (like an electric motor or a stuck relay), which I tracked to the rear left quarter (driver's side), just above and behind the rear wheel well.  Venting the gas cap did nothing however turning the key on and off without starting the engine stopped the noise.  In doing this the electric fuel pump comes on, which is a different sound than the humming I heard.  This had happened once before, about 2 months ago.  The dealer suggested today that I simply monitor it until it became more frequent so they would have a better chance to track it down.  Anybody heard of this before?  What was it?

Thanks

First guess would have been fuel pump. But now I'd suggest that you simply monitor it until it becomes more frequent so the dealership can have a better chance to track it down. 

I have the same issue as ET 2013. Same circumstances -- car parked in garage for at least an hour. Same location for source - underside of rear driver's side. Have detected noise when letting the dog out at night several times. Noise shuts off after 10-15 minutes. Have not sat in cold garage (here in Maine) for an extended period to see if it come back on again. Any ideas?

Fuel pump was the first thing that came into my mind as well.

it thinks its leaking gas so is pulling vacume on tank the new cars moniter gastank vacume at all time even not running so probly normal

We bought a 2014 Outback in August, and on two occasions have heard the same humming noise coming from the driver side rear wheel well when car is off. The car is garagedd. Might be happening more, but heard it these two times. Wondering if there's been a solution to this.

I suggest that you print out this thread and bring it to a service manager and ask that they get an answer from SOA.

They are generally responsive. Ask to make an appointment with district manager if they baulk.

Show them that yours is not an isolated issue.

There may be a simple explanation... and if so, you deserve one. If not, ask them to fix it.

Thank you. I may have to that, and this a great idea.