Barely audible buzzing sound - car is off?
 

My mom's 04 Impala just had the battery replaced. Previously, the car did not start on two separate occasions (it was intermittent).

Since she's had the car (less than 1,000 miles) it has always made a low hissing or buzzing sound even when the car is off. It seems to come out of the AC/heater vents or somewhere close-by. It is so low that some people cannot hear it. My mom and I can hear it just fine.

We are wondering if something may be draining the battery, which might explain the root cause of the aforementioned problem.

What could be the root of this noise?

My car knowledge is somewhat elementary, so I can't do much, but if there is anything that I can check on my own I will try.

Thank you for any suggestions,

Well first I would check for parasitic draw to isolate that concern.
Shut all doors,remove key,disconnect battery preferably with the ammeter already hooked up in line so when the circuit opens the current already flowing through the meter.( to avoid the initial rush of current through the meter and possibly blowing the fuse!)

(Ammeter goes inline between the disconnected battery cable and that battery post where it was connected to. If you're not sure how to do it,or have no multi meter then have a pro do it.)

Wait a few minutes so all the modules can fall asleep,then you want to see something less than 30 milliamps depending on how many gadgets the car has like onstar,remote keyless entry etc. ( on good GM cars I tipically see 7-16 mA or 0.007-0.016 Amp draw)

If that's good then based on your description of the sound maybe the A/C evaporator core has a slow but audible leak and the seller was "kind" enough to fill the system with freon before the sell? Hope not but there is a possibility.
(I'm not accusing anyone as I don't know where the car came from)

What about a fan of some sort? Like the HVAC or Radiator fan.

Radiator fan would be heard outside and the HVAC blower should be felt blowing air out the vents.

The only tiny fan I know about on the inside of GM cars are called "aspirators" in essence they are inside temp sensors for automatic HVAC with a built in very small blower to sample inside temps. They look like microphone or little speaker.
Can be found on face of dashboard, above head on headliner,overhead console etc. They should not be on with key off.

Kodak, how long after turning the car off can the sound be heard? Newer cars wait a while before powering down their electronics, mine for example whines and hisses for up to a minute. If I shut the engine off with the kill switch then I get a whole symphony of pumps, fans and what not for up to 10 or 15 minutes.

If you can hear the sound hours after turning off, then either something is draining the battery, or something is leaking, as mentioned earlier.

The sound is always present. If we get in the car - even before putting the key in - it can be heard.

Thanks for the feedback everyone. Sounds like the battery and this sound are probably related.

I don't own an ammeter. I think we'll have to take it back in to be evaluated.

My Saab 9-3 had a fan for blowing air onto the temp sensor for the Cabin- this was so the Air con knew what the cabin temp was. It often ran when the car was off.

Worth checking to see if yours has one.

Mine was behind a slot cut in the centre part of the dash.

The easiest way to check would be to disconnect the battery see if the sound goes away. If so reconnect and sit in the car in or under the glove box there should be a fuse box pull each fuse one at a time and see when the sound stops that is the area that is giving the issue. If not found there you might need to do the same with the fuse box that is in the engine bay.

Is the heating and air a digital system or the manual dial style?

Manual dial style.

I'll have to try disconnecting the battery. It just seems that any noise that's present no matter if the engine is on or off, indicates that something is active.

Update:

My initial observation was wrong. The sound is actually coming from the speakers (passenger side is slightly louder); I'm not sure if this changes any of the above.