My XJ Series III V12 H. E. produces a strange, rattle-like noise under very specific conditions: Throttle just gently pressed, driving at highway speeds (80 kph and above), all in all applying slight load, e. g. accelerating just a tiny tiny little bit or keeping a steady pace upwards on a gentle slope, RPMs spanning a range from 2,200 to 2,800. The noise is steady as long as I hold the car on this threshold of subtle acceleration/very light load. Most important is the exact position of the throttle: increasing or decreasing throttle just a little bit will kill the noise immediately. The other conditions mentioned are more fuzzy, but they seem to correlate with the funny noise as well.
It sounds like something was loose, but neither can I find any loose parts (e. g. heat shields), nor would it IMHO be logical for some parts to vibrate under these conditions. One would expect the vibrations of parts to be related to RPMs, not throttle position. Or am I wrong?
Throttle potentiometer works fine from 0.33 Volts up to 4.8 Volts, Voltage increases and decreases smootly while turning the pot. That should exclude the possibility of the engine running lean because of a distorted throttle signal at ECU. Bushings at throttle linkage are new and fine, linkage and butterflies are adjusted to specs, nothing seems bent, advance is set correctly, vacuum lines and system have been double checked.
Here is what the strange noise sounds like:
The rattle is audible from 00:00 to 00:08 and from 01:06 to 01:11 – compare the engine sound without the noise in-between. Sound was recorded with a mic positioned at the top of B bank under the bonnet during a test drive. Sound Quality is not great, but the rattle is easy to hear. Interestingly, when sitting in the passenger cabin, the rattling noise is quite similar to the recording, while normal engine noise is, of course, much more quiet inside the car when compared to the mic situation under the engine bay. Might be just a question of frequency response and sound insulation, but I don’t know.
Any ideas what this could be? Worn Timing chain? Turbulences at the butterflies?
I should add that this noise is nothing new to the car. I bought the XJ in 2020, and it did behave in this manner ever since. But now, since most of the other issues have been fixed, I would like to address this one as well.
And I should also add that the rattle is discernible only as a purely acoustic phenomenon. There is no loss of power related to it, no shaking whatsoever. If you could close your ears, you wouldn’t notice it at all.