3 metallic raps

[chrisw99]

I've got a new noise that is driving me mad trying to diagnose. It is a metallic rattle, a bit like a heatshield, but it is always 3 sharp knocks, like someone knocking on a tin can with a spoon. Never 2, never 4, always 3. Somewhere to the front and right of me, probably around the wheel area.

And quite rare, I can go for a drive to listen for it and it never happens.

There seem to be 3 times it happens:
On a straight road at speed just after changing from 3rd to 4th, 4th to 5th etc, literally as I've brought the clutch up.
Driving at a constant speed if the road veers left
At low speed if the car bumps, like a pothole, or just driving off a kerb.

Having had the engine replaced, rattles and the like really make me paranoid, but when I heard it just easing off a kerb once reassured me that it's definitely something metallic and external. I did read that changing gears displaces the engine a bit on it's mounts so I think that explains the first cause above.

But I've been in the bay, underneath the car, tapping everywhere and can't find anything loose or rattling at all.

The strange thing is the fact it's always an identical 3 knocks. If it was something loose I'd expect it to be a bit more random in how it sounds.

Anyone experienced anything similar?

[calnorth]

Is it the CV joint rattle? Drive thrust and play in that area would cause a sort of rattle perhaps clonk or two? And each time drive thrust returns via acceleration.

[chrisw99]

Is there a way I could diagnose if it's CV joint rattle myself?

[calnorth]

My experience with them was that one ran hot...very hot because grease had failed. So I changed the lot on both axles (4). Was a Ford Granada. No clonks/clatter...just red hot.

Looking for damage or wear is something else and I have not done that. Perhaps someone else here has. Other than that I'd say a garage check.