Buzzing sound after turning off engine?

[iaind]

Bit of an odd one. Just bought a 2004 Jag X Type 2.5 - seems to be really good condition and in good mechanical order. One weird thing though - sometimes when I turn the car off, there's a buzzing sound coming from the front of the car. Seems to be either behind the grille or at the front of the engine block somewhere. Sounds like a buzzing relay/solenoid - goes on for about a minute and then fades out. Opened the bonnet this mornind when it happened and couldnt find anything obvious.

The only thing that might be related is the air con - the previous owner said it needed a regas, it's not been warm enough to see how well it works if at all, but doesnt seem to get very cold and doesnt demist the car very well. Compressor engages when its turned on so there must be some pressure in the system.

Any ideas of how to narrow this one down?

 

[OhEsEcks]

Are you sure it's not the radiator fan?

 

[iaind]

Nope, that was my initial thought. Neither fans are spinning when it's happening and if I move them both by hand they spin freely and the sound doesnt change. Both fans do work when the car is running too.

 

[Firestar_3x]

Electronic throttle body?

 

[iaind]

Electronic throttle body?

Doesn't seem to be from that area of the engine, but really hard to tell with that sort of noise. Why would that make a noise after the engine is off though?

 

[Firestar_3x]

Problems with the throttle motor, i.e. its turning or trying to calibrate itself.

In other cars you open the bonnet, turn the igniton on and you can hear the motor going through its range of motions for about 30seconds.

 

[iaind]

I'll have a prod this evening

 

[acemastr]

Mine does this, as does my dad's, it's normal

 

[Firestar_3x]

I'll have a prod this evening

Sounds like this:

https://www.youtube.com/watch?v=DsI8tKgfmFs

 

[iaind]

Mine does this, as does my dad's, it's normal

This is really quite loud, it definitely doesnt sound normal!!

 

[Dr Who]

There are lots of relays/Sensors the ECU leaves live when you turn off the engine to enable a quick restart. They usually go off after a couple of minutes.

A whine or buzz is fairly normal on cars with electronic throttles as well.

 

[paradigm]

Auxiliary water pump?

My old Leon did this after turn-off.

 

[iaind]

There are lots of relays/Sensors the ECU leaves live when you turn off the engine to enable a quick restart. They usually go off after a couple of minutes.

A whine or buzz is fairly normal on cars with electronic throttles as well.

It's definitely more of a buzz or very rapid clicking. It's loud enough that you can hear it on the other side of our car park so although the component making the noise is probably meant to be doing ...something... I'm sure whatever it is is going to be some sort of faulty. I'll see if I can get a recording next time as I'm sure this isnt normal

I'm considering pulling fuses and relays from the underbonnet fusebox next time - if it stops when one is pulled then that'll hopefully tell me what it's connected to

Auxiliary water pump?

My old Leon did this after turn-off.

Struggling to find if it's got one of these - from a bit of googling I dont think it does.

 

[iaind]

Mystery solved!

I had run it for a good while (10 days or so) without the A/C on - trickier than it sounds as the climate system turns it back on every time you change something, so have to be conscious to turn it back off... no more noise! So I knew it was something A/C related, figured I'd get it regassed and see what happens.

Turns out there was reasonable amount of gas left in the system but very little oil so the valve that controls the flow of refrigerant had partially seized. After the regas it still wasnt cold, but left it running for 10 minutes and it's suddenly very cold indeed. I'm guessing the buzzing sound was that valve and adding lubricant to the system freed it up - hopefully permanently.