Mystery sound in my SUV

This is strange as berkeley. I periodically hear a phone notification chime in my Expedition.

It isn't coming from any cars outside and it isn't coming from my phone.

I hear it at the most random times while driving or sitting in the car.

It sounds like it's coming from the second or third row.

I've looked all over and even under the seats. 

Now I'm thinking someone left a phone in here or there's a secret compartment somewhere that I don't know about. 

This is the sound I'm hearing. 

That's pretty strange. 

I would think that a phone would be dead after a couple of days though. 

In reply to Toyman01 (Moderately Supportive Dude) :

None of the typical Ford chimes. This is Samsung hence why I think it's a phone.

Isn't there storage in/under the middle seat in the second (or 3rd) row on these?

Do you have a Samsung phone?
Is it alerting to let you know about new Bluetooth hotspots that may be driving by or otherwise trying to pair with another device?

Good note. 

I will make sure to turn my phone off next time.

  - Your Stocker

In reply to Brett_Murphy (Ex-Patrón) :

I do have a samsung phone. Today I did an experiment and cut my phone off. It still happened. I didn't about an hour driving the truck today and sat in line at my son school for about 30 mins. The noise happened 10 times. 

In reply to mtn (Forum Supporter) :

Looking now lol

So, when I bought the truck the previous owner left a decent size subwoofer in the back but it wasn't currently wired up. I took it out and put it in storage because it took up so much cargo space. 

I know this knob was installed for the sub:

And I did some digging just now and in the cargo area there's these wires, that I assume are for the sub:

Which leads to this, hidden in the cargo panel where you'd access the lift pump for the tailgate:

Brand is PowerBass

So maybe the noise is from some left over audio device hidden in the truck somewhere? I don't know. 

How long have you owned it? Did the noise just start?

In reply to Steve_Jones :

2 months. I've only heard it randomly over the last few months. 

Took the rear cargo area completely apart. Didn't find anything. I did find some Miami Dolphins coach passes under the cargo area. So i think one of the previous owners was a coach. 

Does anyone else hear the noise?

In reply to Streetwiseguy :

LMAO

Plug in the sub. Fixed.