I have a 1990 Jaguar XJS Convertible V12 with 61,000 miles on it.

As of last night, there's a weird knocking / clunk coming from my car. You can only hear it inside the cabin. You can't hear it coming from the engine. It sounds like it's coming from the dashboard. Accelerating or decelerating doesn't seem to change the frequency of the knock and you can't hear it from the engine compartment, even with the hood up (you need to be inside the car) so I don't think it's coming from inside the engine.

Changing gears / going into reverse or park doesn't affect the frequency either. It seems to start making the noise once the car has warmed up to normal operating temperature and it sometimes will stop for a minute. It also doesn't seem to be making the noise at a regular interval but it happens every few seconds or a little longer than that.

Listen to the sound: http://cdn.wrongplanet.net/audio/jaguar_xjs_knock.mp3

 

Is that a vent flap (for changing flow from dash to floor etc)? Does changing the climate control settings have any effect?

 

Yes, it appears to be related to the climate control settings.

 

I'm not experienced in this area and was hoping others would jump in. My guess is that you have a vacuum leak or solenoid working intermittently. We'll need to know how far you are willing to go yourself. See the photo--dashboard passenger side and centre dismantled to get at the parts. Ignore the yellow circles--the photo is not mine. I think you need to look at the two gold-coloured solenoids at the top left of the photo, and the vacuum lines attached to them.

 

I know it’s a stretch due to the age of this thread, but did the OP get this figured out? Having the exact same issue - a single knock behind the dash after car had warmed up (almost sounds like someone flicking the dash with a spoon or something like that).

I suspect climate control as well since I just replaced the heater core but haven’t had a chance to mess around with this just yet.

 

If you have a metallic knock then it is very likely to be a flap or a matrix (could be heater or evaporator) whose foam anti-rattle insulation has gone home.

 

But wouldn't that present itself as a rattle or tapping sound - this is one single knock over a half hour span, no more, no less. Seems oddly consistent. I'm trying to think of what happens (or what moves) in the climate control system when the car gets to temp. This is happening with the fans off and the temp switch set to 85. Like maybe there's a spring or vacuum involved that's suddenly been released. I'm obviously going to do some digging, I'm just hoping someone may be able to provide a couple focus points.

 

I thought that you said it was a metallic sound? perhaps you can describe it more fully? Is it a knock that only occurs once evry half hour? I am confused as the exact situation.

 

Sorry if I wasn't more specific: when the car gets up to temp (or thereabouts, it's usually after 20 minutes of driving but maybe less, I haven't timed it), I'll hear a single knocking sound from behind the middle of the dash. The sound can best be described as flat - no ringing or scraping or rattling. Almost like someone tapped the butt end of a screwdriver on a concrete floor. Nothing bad happens afterwards, it's just the sound by itself. And frankly I could live with it due to its frequency, but I need to find out if something is under stress and needs adjusting/replacing.

 

What is the frequency of the sound?

 

I've not driven the car more than 20 minutes at a time as it's still somewhat in the project phase, but the sound typically happens once after the car gets to temp, and doesn't happen again unless I shut the car off for a *while and then start it back up. After starting it again, it may only take a couple minutes for the sound to reoccur. I've never heard in twice in one drive unless, again, there's a shut off period at some point.

*A "while" varies, but usually no more than an hour maybe.

Can you think of a lever, spring, or vacuum-driven operation in or around the dash/center console that always occurs when one of these cars reaches operating temp?

 

If you have the A/C on, dies the noise coincide with getting morecair out of the center vent? The flap for it can stick, and release suddenly and make a thunk.

 

I've not really used the A/C since the car has mostly been on jack stands since I got it right before winter. The noise occurs with the temp set to 85 and the fans completely off - I've not had enough time to see if it occurs under other conditions.

Though I am curious - I'm new to these cars and I don't see a switch or dial to direct air through certain vents. Is this done automatically based on fan speed?

 

It's done sutomatically by the climate control system, based on set temperature and various sensors.

 

That noise is just the blending vanes and possibly the fan flaps getting themselves in order. Nothing to worry about, caused by the fact that the foam anti-noise stips have gone south.

 

Bending vanes?

 

bLending vanes: these are used to adjust the mixed exit temp of the cold and hot air sources into the cabin. They are vac controlled on earlier cars.

 

That sounds very, very similar to a similar issue I had with my car: the loud dull knock, once, after the car was running for a bit. The aircon was indeed the culprit, as Greg notes. In my case, it was a vacuum leak, if I remember rightly. Pressure was building up, then suddenly released. Or the other way around.

 

It would be worth checking the vac feed from the A bank manifold, rear end, to the vac reservoir to see if the non-return valve is working and to see if the vac line itself is holding vac, and the vac reservoir is too.

 

A video would be great. Describing it is almost useless