Hello to you All.
I had a look with the search tool on this forum to try and find a similar subject but couldn't so here we go.
1997, XK8 conv.
I get a brief mechanical rattle / knocking noise when the engine revs fall from any blip on the throttle.
It doesn't do it from a cold startup but will always do it after approx 2-3 mins from cold start.
If I gently let the revs drop from any throttle position, it also doesn't do it.
It only does it when I take my foot off the throttle completely and quickly.
In gear and while driving you never hear anything. It only does it in idle.
The sound is heard louder from under the car (head out of door under side of sill) while tapping the throttle.
The engine starts, idles and drives perfect with no misfires and the tensions and guides are all 4th generation fitted last year. engine is at 53K miles from new.
This is my first auto car so I have limited knowledge about the components between the back of the engine and the gearbox but I would guess at it being in this area.
Can anyone shed some light on it for me?
Many thanks in advance.
Wayne

 

Check the transmission mount and the heat shields first. A broken catalyst core can also cause a rattle, but look for external sources first. Exhaust hangers?

 

RJ is probably right but wanted to note how easily a faulty idler pulley can sound like this.

 

Hi Everyone. I thought I had traced this to a rattly CAT and have ordered some new ones and also a stainless cat back exhaust for it.
After coming back from having 4 new tyres fitted I have seen the oil pressure message come up and the needle drop to the first notch on the gauge.
I've never seen this happen before and am of course don't want to start it.
Has anyone seen this before? or is it likely that the rattle could be the oil pump?
Thanks for any insight.
Wayne

 

It could be a chain tensioner blew apart. The secondaries would probably result in the chain skipping teeth and a valve/piston collision, but a primary could cause loss of pressure without catastrophic failure. The primaries are hard to examine without a borescope but I would start there. Anything more complex will probably require removal of the engine.

 

?

 

Hi RJ.
I installed all new 4th gen tensioners and guides last year.

 

New doesn't mean infallible.

 

Loss of oil pressure is fatal. Did you have a garage do the work? I would remove the cam covers and inspect the chains, guides, and tensioners to check their work. Any codes??

 

UPDATE.
OK so I installed the new CAT's today as well as fitting the new quicksilver, resonated exhaust system and it sounds great....Also no more rattling noise, it was definitely the offside CAT monolith that had fallen to bits and was shaking around.
Also renewed the oil, filter and pressure switch which has cured the issue with the low pressure warning.
Thanks to everyone for their recommendations and support......I hope this thread helps someone in the future.

 

was the loss of oil pressure a brief dip at idle after driving for a while. did it seem to correct itself after a second or two and/or upon lightly touching the throttle?

if so, you might have sheared the oil where the viscosity improvers were destroyed and could not keep it at the rated viscosity. you see this when you get old broken oil hot.

if you do not yet have it, install Real Gauge with the oil pressure option. this will tell you if you have correct pressures and at what temperature.