My 1999 XJ8 (123,000+ miles)started making a thrumming noise recently
when on the road.
It sort of sounds like the noise you get when running on the road strips
that alert you when you go off the side of the road
or are approaching a stop sign.
I don't know if they have these strips on the roads in other countries.
But, they do provide a nice audible alert to the road conditions and your
car's position on the road in the USA.

The sound only happens when I am accelerating or applying power going
up a hill. As I said it is a "thrumming" or a "Zrooming" sound.

At first I thought it maybe a noise created by tires and pavement.
But it only happens under load.
Then I recently read the "Timing chain Tensioner failure" threads and
the sound described for that failure is a "rattle".

Starting up the engine it sounds okay with no "rattling" or "thrumming"
noise. It idles quietly and even revs okay. Only during road travel
does the noise appear.

Due to a small oil leak in the cam cover I am planning on replacing
the seals soon and I will inspect the tensioners then.

At 123K miles is my Jag on borrowed time? Or are the tensioners a
hit or miss failure item?

Regards,
Bob - POR - Press On Regardlesss

 

A 4.0 V8 of that era is past being on borrowed time!!

Your noise may well not be the tensioners but not the first sentence either way.

 

If you aren't sure about the tensioners, remove one of the cam covers and have a look to see if the secondary tensioners are of the plastic or metal variety.