This doesn't happen all the time, and sometimes it is difficult to replicate while other times I can. Basically, when it occurs, there is a noise which seems in between a click and clunk as I take off from a full stop.

The weird thing is that the origin of the noise seems to move. Sometimes I'm certain it is from around the left rear wheel, while others I think it is from the front of the transmission.

I put the XJS on jack stands last week and tried to locate the cause. I couldn't find anything loose or out of sorts (but then I'm no expert).

After searching the archives, I may have a guess. I recently replaced the transmission mount, and in doing so, I had to drop a portion of the exhaust. I didn't realize until reading some threads that aligning the exhaust is a bit of a black art.

Is it possible that I did a poor job in returning the exhaust to position, that it would make noise sometimes in the rear and sometimes more forward? Would it make noise on take off?

Thanks

 

Either the exhaust, or also a possibility is the gearbox tail making contact with one of the fixing brackets. The rearmost one, if the box and both brackets are not perfectly aligned, can make contact with the tail.


The exhaust is of course just as likely a cause. My problem is that, having a 2 post lift (not grumbling, mind) the car is not on its wheels when I adjust the exhaust. And the way the exhaust hangs changes substantially when the car is back on it's wheels - just the same if up on a jack. Check for the pipes contacting the gearbox sump, the rear cage/calipers where it snakes through, and the exhaust tunnel sides.


Greg

 

Thanks Greg, but not sure I fully understand. Do you mean I may have not done the transmission mount correctly?

 

Here are the diagrams of the two brackets I meant. They both can float a bit and have elongated mounting holes. It is very easy to fit them so that the propshaft is too close to the second diagram's bracket. It is quite easy to fit the first diagram's bracket so it throws the box out of line when tightened up.
The exhaust is a more likely candidate, but the box can be a sod because of the brackets' play. That is why I thought out a fitting system so it could be fitted and adjusted dead centrally by tapping the bracket as it was tightened. I will try to find my post and add it when I do,
https://www.jaguarforums.com/forum/x...itting-145478/
Greg

 

Greg when I did the trans mount, I used your easy safe method, but perhaps I didn't do the alignment properly.

 

A little bit more information.

This afternoon I had to run several errands, and all those short trips were a good opportunity to listen carefully and do some trial and error.

My best guess is that the noise comes from the exhaust joints (front was a sleeve like one and the rear a "olive") I had undone to drop the exhaust when I did the the trans mount. Which explains why it sometimes sounds like it is coming from the rear, and sometimes from the front of the transmission.

Additionally, when I first drive the car when it is stone cold, it doesn't make the noise. Once the exhaust heats up, the noise starts.

FYI, this was the first time I ever touched an exhaust system, so I admit a large amount of ignorance. When I put the exhaust back together, I didn't use any compound or sealant, because I didn't see any used when I took it apart.

Should I have used sealant or something? Did I do anything else stupid?

 

Mac, tighten it up, and if ANY part of the pipes are closer than 3/4 of an inch to anything solid, adjust so they are not. Be very attentive to the rear cage and ensure the pins on the convoluted cage section of exhaust are located into the cage top, and that they have their insulators etc all fitted. Just keep trying until the noise goes! If you slip a garage a tenner and can get the cat up on a four post lift when it is still warm, so the car is on its wheels, you will see where the trouble is.
Greg

 

Another possible cause while not common can lead to the "at take off" symptoms is a deformed safety strap on radius arm to body mount. I have seen them deformed inwards to the point where they only just clear in a standing position but when power is applied the radius arm moves slightly forward making contact with strap.

 

Just looked at the exhaust section of XJS Workshop Manual, and it says to use exhaust sealant. Should I pick some up and put it on the joints I disturbed?


I'll admit that I'm not sure what pins you are referring to, and I don't recall them from when I did the mount. I can't find any mention of pins in the Workshop Manual, and I looked at all the illustrations and I'm still a little lost.

Will be getting under the car in a couple of hours, so any more insight is appreciated.

 

What he is referring to is there are exhaust hanger pins where the exhaust goes up over the half shafts in the rear, easy to miss them as well they have rubber insulators that could fail.

 

Thanks. OK, I think I know what you are talking about, and IIRC the hanger rubber was in good shape, but will double check when I'm under there later.

 

Quick update

Yesterday I R&R'd the same exhaust sections I had when I did the trans mount. This time I did a better job of positioning and aligning the pipes, and I put exhaust joint sealant on the disturbed joints per the workshop manual.

The right side had two issues, it was too close to the trans fluid pan, and the rear olive joint was too close to the cross brace. Also found on the left side that one of the nuts holding the strap that goes from the front of the trans to the rear cat had a broken lock washer and had come loose.

I hate when my own incompetence leads to having to do things over...

Much quieter now, but not totally noise free. If approaching a stop where a right turn is required (where you start the turn as you approach a stop), there is a faint click from the right rear when you take off from the full stop. If it were a coupe or the top was up with windows closed, I doubt it could be heard. Doesn't do it on a left turn, or going straight.