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[Auto Addict]

Someone has bumped the back of my car, by the exhaust.

On tickover everything is fine, no noise

Put the air con on, sounds like the back box is blowing.

Open any door, inc. the boot with air con on, noise goes away.

Any ideas?

[Markjay]

backbox mounts... box ressonating? Or rear pipe cracked where it goes into the box?

One way to check for leaks, is place your hand firmly over the exhaust while idling (seriously - it's harmless but use gloves if you are concerned), you should be able to 'choke' the engine. If the engine does not stall with the backbox completely blocked, then you have a leak somewhere, and you might even hear wher is is blowing from...

[Auto Addict]

backbox mounts... box ressonating? Or rear pipe cracked where it goes into the box?

One way to check for leaks, is place your hand firmly over the exhaust while idling (seriously - it's harmless but use gloves if you are concerned), you should be able to 'choke' the engine. If the engine does not stall with the backbox completely blocked, then you have a leak somewhere, and you might even hear wher is is blowing from...

Yes, but why would it sound as if it's blowing with the air con on, and all the doors closed?

Open any door or the boot and the sound goes away, or if I turn the air con off.

[Auto Addict]

I'll go any do what you suggest MJ, back in a tick.

[Auto Addict]

Tried that MJ, engine doesn't stall, but I think that's because I can't get a proper seal with my hand.

What I think is, but I need to get under the car to do a proper investigation, with the air con on, there is more vibration from the exhaust, and it could be vibrating against something, making it sound like it's blowing.

Strange it disappears when you open any of the doors though, even the boot

Some PRATT has obviously reversed into the back of my car whilst I was parked, and driven off.

They could have caught the back of the exhaust and moved something.

[x25xe]

Just two thoughts - probably way of track though

With air con on, the engine is under extra load causing more vibration than normal.  If the exhaust system has been moved then it is highly likely that it is resonating against something like a heat shield, handbrake cable etc.

Don't really know why it disappears with a door or boot open - maybe (and this is a long shot) with the door or boot open, the "box" effect of the car is removed and hence the sound is no longer amplified?

[Tony H]

Just two thoughts - probably way of track though

With air con on, the engine is under extra load causing more vibration than normal.  If the exhaust system has been moved then it is highly likely that it is resonating against something like a heat shield, handbrake cable etc.

Don't really know why it disappears with a door or boot open - maybe (and this is a long shot) with the door or boot open, the "box" effect of the car is removed and hence the sound is no longer amplified?

With aircon on tickover is increased, try slowley increasing the revs and see if the sound changes or dissapears, sounds to me like the exaust is in contact with the underside or similar.

[hotel21]

Possibly wayyyy off the mark here, but are there not air flaps/vents in the boot somewhere to allow free flow circulation of internal air?  Bump may have dislodged or done summmat to them.  When aircon is on and door open, effectively bypassing the vent affair in the boot??   :-/

Like I say, probably a long way off but its always worth a wee look?

HtH

B

[Marks DTM Calib]

Yes, check the airflaps in the base of the boot right hand side pocket.....

[Auto Addict]

Been out in it tonight, everything seems back to normal

Thanks for the advice guys.

[Markjay]

I can just picture you with your pyjamas and torch under the car...  

All's well that ends well  

[Ken T]

I know a bit looked down on, but if you have a tow bar fitted, then anyone bumping into you is going to do a lot more damage to his bumpers than yours, and people tend to notice big lumps of steel pointing in their direction.

[Auto Addict]

I can just picture you with your pyjamas and torch under the car...  

All's well that ends well  

Pyjamas & torch?? Nightshirt & candle if you don't mind

[Auto Addict]

Interesting, nothing blocking the air flaps in the boot.

If I blocked off the air flaps I could re-introduce the noise from the back box.

Driven it around all day with no problems.

Strange........