2006 XJ Base.

This started the middle of last week and has happened maybe 5 times since then. When I turn the car off, there's a thump on the back of the car. The thump doesn't happen all the time and it varies in intensity. Sometimes it's rather loud like the sound of an underhand throw of a golf ball on the bumper, sometimes it's more like a muffled thump like a golf ball wrapped in a wool hat hitting and sometimes its barely perceptible. I can't tell exactly where it's coming from, whether its in the transmission, wheel (emergency brake?) or some sort of engine backfire coming out of the right exhaust pipe.

There are no error codes/lights coming up on the dashboard.

Three weeks ago the car went through it's 100,000 mile checkup and the repair coordinator came to me, shrugged her shoulders and said "the car is in perfect shape".

Has anyone encountered this sort of thing? Do you have any thoughts on what it might be?

 

Grasping at straws here since I didn't hear the noise. Do you have the sound system on when this occurs? Could the sound be coming from a rear speaker? Just a thought.

 

It's unlikely, but I'll check. I'll make sure to turn off the sound system before I turn off the car. It sounds further back, like it's rear right wheel, or rear right exhaust.

 

If you can get it to act up, might be worth it to find a place with a drive on lift, like are used for alignments. Then someone can sit in the car and turn it off, and someone can be underneath to see if it might be a exhaust bracket or such. Reason I suggest drive on lift is, suspension would not be hanging down. Would be more like sitting on the road when your problem happens.

 

Hi FRGIII2014,

Two possibilities come to mind:

1. If your Electronic Park Brake is set to automatically engage when you turn off the car, you may be hearing the right rear EPB caliper engage. It might be making noise due to some resistance in the cable or lever that needs to be cleaned and freed up.

2. Could it be a noisy relay in the Rear Power Distribution Fuse Box, which is mounted at the right rear of the trunk behind the battery?

Cheers,

Don

 

My power antenna won't go up or down because it's old and sticks too much, but of course the electric motor tries to extend and retract it anyway. It makes that same thumping noise on start up and shut down because the antenna motor and gears are spinning, but the antenna isn't moving, and all that is located in the right rear side of the car. With the cold weather we're having, I'll bet thats what it is.

 

Hi davejdoyle,

Is your car an X350 or an earlier model? On our X350, the antenna is part of the wiring in the rear window glass, and its signal is processed by the AM/FM Antenna Amplifier, which is mounted in the cabin at the left-hand D post. There's no electrically-powered antenna/aerial in the rear right fender as there is on the X308 and earlier cars.

Cheers,

Don

 

Hi Don B,
Its not an X350, I have an 1999 XK8. FRGIII2014 had said in his initial post that he has a 2006 XK, and I think that has the same power antenna as mine.

 

I mistyped. I have both cars (XJ and XK), but the thumping problem is with my XJ (350).

I turned it off after coming home tonight and before I did, I turned off the radio/cd and put my foot firmly on the brake and there was no thump. Tomorrow I'll turn off the radio/cd and not have my foot on the brake and see if that makes a difference.

 

The most likely issue is a blocked fuel tank breather. Do you get an air flow noise when you take the fuel cap off ? The noise you hear is the tank re-pressurising. If it does it too often you will have a boot (Trunk) full of fuel. Get it checked asap

 

Always make sure the body is completely deceased before placing it in the trunk or thumping will occur.

 

While it is true that I bought the car in Chicago, I no longer live there so this probably isn't the issue. Good assessment though.

 

Older Lincoln Continentals more susceptible to this problem, than your Jag.

 

I checked the pressure in the tank tonight and no sound of air flow...sounded good. That's a relief.

 

Caddys too. In GoodFellas, Henry's car is not just any Cadillac, but a “Special Edition” Coupe de Ville Phaeton. It had one hell of a noise coming from the trunk. Until they fixed it ...

 

As far as number 2, it really sounds like a golf ball dropped from 6 feet in the air when it's loud, a bit more subdued other times. It doesn't sound as much like an electrical snap, it's more like an impact.

As far as number 1, since it sounds like an impact, it seems to be narrowing down to something with the brakes. When I first heard it, I thought a rake had fallen off the wall and hit the side of the car. This morning and this evening, I kept my foot on the brake when I turned the key and I didn't get the sound.

I may get a chance this weekend to get it on a lift and take a closer look. There doesn't seem to be anything mechanically at issue with it.

Thanks again.

 

Sorry I can't help, but I'll move this thread back to the XJ350 forum for you.

 

Hi FRGIII2014,

Have you tried manually engaging and disengaging the EPB to see if you get the sound?

Don

 

Yes, no sound.

Strangely, at first I tried things to prevent the thump based on recommendations from this forum. The thump stopped. Then I started trying things to see if I could make it happen again so I could take it to the local dealer to be able to explain it to them...and I can't get it to happen again. It's been four or five days since it's happened.

I'm going to go get my tires rotated this weekend and while it's up on the lift, see if we can get a look under there to see if there's been some sort of cable or hose that snapped loose.

 

The lady who runs the Jag part of my local dealership's repair area said that it was the brake and that the sound wasn't unusual. It only happens about once every 10 times I turn the car off and engaging the EPB before I turn the car off seems to significantly reduce the likelyhood of the sound. It still seems unusual though, because I've had the car for 7 years and never noticed the sound until this winter.