I'm in the third year of owning my 2007 XK and ever since I bought it I have been trying to track down a knocking sound. The knock starts after the warm-up period then generally sounds less significant after more than ten miles of driving. It is not noticeable above 1500 RPM.
Hopefully, this information may help someone plagued by the same weird knock in the future.
I've attached a short audio clip.
In my efforts to track it down I've searched forums, asked mechanics and friends etc. Usually, most opinions have been lifter knock, but this sound is loudest low down in the engine at the rear and can be heard more from the underside of the car. After using my trusty wooden dowel and my ear some months ago I found it to be loudest on top drivers side of the bell housing.
I just had the transmission pulled and it turned out to be a warped flex plate hitting the engine casting web in the image below. The red circle shows the casting web that is being ground by the flex plate. The blue circled casting web is how it should look. The flex plate is warped by about 2mm.
I'm having the flex plate replaced

 

Certainly one of the more unusual causes! Good to hear you finally found the cause.

Is this (arrowed in orange) a crack?





Graham

 

I wonder how the flex plate got warped in the first place??

 

Thanks for pointing that out Graham.
I believe it's a casting line, but it does look odd in the image and I did question it too when I was editing the photo.
I took this photo myself, so I saw it first-hand and did not notice it as an alarming crack feature at the time, nor did the mechanics highlight it as an issue.
If you zoom into the machined face at the top of it, you'll see it does not run through the material at that point. The other end of it runs all the way up the web to the upper right of the blue circle and it's clean with no sign of an oil leak.

 

That's a good question I've pondered too.
Since it's only seems to be wearing on the casting web on the left side, it might suggest excessive misalignment that's stressing the flex-plate.
I'll be switching out the old transmission and torque converter for used, but lower mileage ones in this repair.

Other symptoms to note are:
1. Knock starts just before revs go from 1500 downwards at the end of cold start warm up period.
2. Knock intensity seems to change when shifted into gear.
3. Knock apparently disappears at higher RPM.
4. Knock occurs only when transmission does not load the engine. Acceleration and deceleration there is no knock, but idling or floating the throttle there is a knock.
5. Knock improves over the time the engine is running - Over a 10 or more mile drive, it gets quieter.

 

That would be a bunch of misalignment. I'd put the old flexplate up in place without bolting it in and spin it on the end of the crankshaft. That should give some ideas about where things are off. Has the transmission been out of the car previously?

 

“Other symptoms to note are:
1. Knock starts just before revs go from 1500 downwards at the end of cold start warm up period.
2. Knock intensity seems to change when shifted into gear.
3. Knock apparently disappears at higher RPM.
4. Knock occurs only when transmission does not load the engine. Acceleration and deceleration there is no knock, but idling or floating the throttle there is a knock.
5. Knock improves over the time the engine is running - Over a 10 or more mile drive, it gets quieter.“


Could I ask what mileage this began to be noticeable, and current mileage?
Thanks.


John

 

Since it was really doing it when I bought it, I have no idea when it could have started. I can tell you I've been confident enough to drive it for 12K miles since I bought it Feb of 2019 and the sound on somedays is quieter than others (I assume because of temperature changes).