I've never heard anything like it. In fifty years of spannering I've never heard anything like it. It is coming from either the brake servo, or the bonnet airbag, or something below them. When it's doing it, it can be heard in the driver's seat (RHD, remember) when the engine is idling.

The sound is like a cross between a sizzle and a hiss. not very loud.

My first thought was that it could be the cooling system either venting excess pressure or equalising pressure after cooling down, but removing the cap made no difference. Same goes for the brake master cylinder cap, and the washer fluid filler (clutching at straws here...).

Starting the engine and applying the brakes to exercise the brake servo makes no difference either.

It seems to be temperature-dependent; it happens to my knowledge when the air temperature is rising during the day, or falling in the evening. It was doing it this afternoon, when the sun fell on the car after it had been in shadow for much of the day. It had not been run or moved for 24 hours.

Please God, it's a known fault, and somebody on here has had it and can tell me what cured it! Please help....

TN

 

I am wondering whether you might have an engine coolant leak which is dripping upon a hot exhaust manifold?

What is the engine coolant level in the reservoir, and is the level being maintained over time or is it dropping slowly?

 

Sure it's not the little HVAC cabin temp monitoring air pump? Doesn't sound like it but if you haven't experienced it you might be surprised.

 

The car had not been run for 24 hours, and the coolant is just fine; it's not that.

 

Would it run if the ignition had been off and the car locked for 24 hours? If not, then that's not it.

I know it's difficult to describe sounds, but I could also describe it as a faint, irregular, rapid popping sound, but with not much pressure behind it.

It's definitely not hot metal ticking as it cools down, either.

 

Video (or probably Audio) might help.

 

Yep, it could very well be. The car kind of wakes itself up when the key fob gets close to it. Sometimes I'll hear the HVAC pump running when I get in the car. I used to think it was part of why my battery would die. Other times I won't hear it until I shut the car off.

Anymore I try to religiously remember to lock the doors when I get out as that puts the car to 'sleep' more effectively and quicker vs. just shutting it down for the night. I also found out that it won't go to sleep with the hood open so it must go on the charger those nights.

 

Car hadn't been run for 24 hours, doors locked, and the key fob was in the house over ten yards away. I don't think that's it.

TN

 

You have not left the radio on low have you. Just a thought

 

Have you heard the HVAC cabin air monitoring pump when your in the car? If not I spect that is the noise you are hearing. Threw me for a bit of a loop when I first heard it.

 

Car hadn't been run for 24 hours, doors locked, and the key fob was in the house over ten yards away. I don't think that's it.

TN

 

I don't know why people think that because it isn't true. The car has no idea you are there until you try and open a door or the boot or press the unlock button.

The official description is:

When either door handle is pulled or the external trunk release button is pressed, the KVM activates the low-frequency antenna at the point of access (either door or trunk). If the Smart Key is within 1.0 meter (3.3 ft.) of the active low-frequency antenna, the Smart Key will transmit a high frequency signal which is received by the antenna above the roof console. If the Smart Key's code is recognized, the vehicle will unlock the doors and disarm the security system when accessing a door. If the trunk is being accessed, the trunk will release but the alarm will remain in an armed state.
I was surprised last time you mentioned that so checked on my 07 XKR and it went to sleep as expected after around 30 minutes so I wonder why yours doesn't.

 

Car hadn't been run for 24 hours, doors locked, and the key fob was in the house over ten yards away. Could the HVAC pump have been running under those conditions?

TN

 

It probably would, but it's beyond my technical abilities, I'm afraid. I could probably take a video with sound (I have an iphone 5S), but posting it is a closed book to me.

TN

 

If the car was still locked and you had made no attempt to unlock it or open the door then no. The fan should stay on for a maximum of approx 30 minutes and will shut off a lot sooner if the car is locked.

 

If you have a YouTube account you can upload the video to that then share a link to it in your post here.

 

Why do YOU have to have the technical abilities? Get somebody else who does.

 

Don't think I'll bother. It happens occasionally, but with no pattern to it. The car has done five hundred miles since somebody first noticed it, and continues to drive entirely normally. I'm not worried; I'll live with it. Maybe it will stop of its own accord...

 

The suggestion has been made that the sound could be from the small aspirator fan for the HVAC; the fact that the car had not been running for 24 hours, that it was locked and that the key fob was in the house is irrelevant. You see, simply pulling the door handle to open starts that little fan running and it will continue to run for some time after the engine is shut off. If you hear the noise from the driver's seat then you have started that fan running by opening the door. Some of the fans are noisier than others; if yours is particularly noisy it can be replaced; the fan is behind the small grille on the dash just to the side behind the steering wheel.

 

The car had been stood, locked, untouched, for 24 hours. Unless the Phantom Door Handle Puller had been round, that's not it...

I presume the fan runs all the time when the car is running? The noise doesn't...