Open diff - new rattle

[Paul D]

Last month I did 2000 miles in 2 weeks on a trip to Scotland from the South coast. 25 miles before I got home a new rattle from the rear of the car suddenly started on a left hand corner and has been present ever since.

All the usual diff-related noises haven't changed, but I now have a rattle that increases in frequency with engine speed (not wheel speed) and is only present on light throttle; it goes away when coasting (whether in gear or not) and on half throttle or more.

I've had a second opinion from my mechanic friend with the car up on ramps and it is definitely from the diff (not a CV joint as was my other thought).

Car still drives fine (for the moment!). 2008 1.6 K series Roadsport car, open diff.

Any ideas what might make a 'loose' rattle sound when it fails in a open diff? Just thought I'd see if there was any experience of failures on here before I get it stripped down to see what's what, although I don't ever recall reading about open diff problems, just LSDs.

Thanks for any insights.

Edited June 21 by Paul D

[Paul D]

Forgot to add that when up on the ramps with the drivetrain engaged, noise seemed to be from the front of the diff.

Mechanic friend, with no particular Caterham experience, suggested stiff UV joint or pinion bearing issue. "I don't ever see anything this old, modern differentials have usually exploded before I see them" 😄

[John Vine]

Could be a failing rear universal joint.  Does your propshaft have grease nipples?

JV

[Wrightpayne]

I’d tightness check everything on the drivetrain - from engine mounts backwards.

The lower bolts on the Sierra diff are known to shear. They are a modified imperial bolt as they have a 1/2 inch shank to fit the chassis bush and a metric thread to fit the sierra diff.

Also check the dedion tube for any cracks particularly around the shock mount and a frame mount.

[Wrightpayne]

On a friends seven, which he purchased second hand, the lower diff bolts had normal metric bolts installed so there was about 1/2mm play within the bush. Not ideal on a 2L Paul Exon tuned VX!! 

[Geoff Brown]

Probably only two causes: rear UJ loosening up or CWP out of adjustment. Either could be catastrophic if failing at speed ! 

But stating those issues,  I know someone who while returning through France to UK  experienced a rattle from the same area that increased during the journey. Made it back to Blighty but when he looked underneath the rear UJ was only just holding together !  So the UJs are pretty tough.........

[Paul D]

Thanks all, some good suggestions to follow up.