Odd sound with the engine off but key in ignition and power on

[16g-95gsx]

So I was reinstalling the sunroof tonight, so I had to turn power on to slide the front glass back and forth. What I noticed is that I hear a very very faint sound that sounds exactly like a car alarm coming from what appears to be the engine bay. It goes away the moment the key is out of the car. Any idea what it may be? Perhaps it is the alarm, and if so where is the siren located as I can disconnect it to test if it goes away?

[wpoll]

Not sure if yours is the same as mine (I don't see what model X5 you have) but on my '05 the exterior alarm sounder (siren) is in the cavity next to the brake master cylinder/reservoir - below the wipers.

[16g-95gsx]

Yep I see the one you're talking about, it definitely isn't coming from that. I swear I hear it most prevalently when I lay under the engine. It honest to god sounds like an extremely faint car alarm going off. In fact I opened the garage just to rule that out but it's most definitely coming from the engine bay area. I work on cars al the time so if it was a common sound I would have identified it, this is something unusual for sure. Where are the standard horns for the X5 located? Again I hear it strongest from underneath the engine, albeit it's still faint there. Very strange

[16g-95gsx]

I have a 2005 4.8is if it at all matters

[wpoll]

I've heard the electronic ignition systems in modern cars give off faint high frequency "song". The high frequency pulses in the individual coil packs on top of the plugs cause the materials in them to oscillate a little, creating a high pitched sound. You can usually only hear this with the ignition switched on but the engine not running - too much noise otherwise. I would image that eight of these things all harmonising would be an interesting sound.

[16g-95gsx]

This isn't a constant tone. This sounds like a car alarm where it has a cyclical high and low that's on the order of a 1-2hz frequency. It sounds strongest from underneath near the oil pan/ trans as though it's coming from down there. Imagine a car alarm but 1/10th the sound, faint enough that you wouldn't hear it if it wasn't in a garage and you had good hearing. Both my roommate and I heard it and reacted the same way "what's that?"

[wpoll]

Very odd. I have an aux. water pump on my diesel that fires up with the key on - part of the aux. heating circuit - but it's low down at the front behind the radiator.

Transfer case actuator motor cycling?

[16g-95gsx]

I suppose that's work trying. I could just disconnect it's harness.

[16g-95gsx]

http://4nmuscle.com/wp-content/uploa...0/IMG_8627.mov

Hopefully that works. For reference that is a video from underneath the X5, facing towards the front, laying underneath the transfer case. Disconnecting the transfer case solenoid made no difference. It's quiet so be sure to turn up the volume.

[wpoll]

Yeah, that IS a weird sound - and I can see how you relate it to an alarm.

I got nuttin'....