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RogerB
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Thanks for such prompt responses. Some of the suggestions I have covered. I will investigate other suggestions and see if I can upload a U tube recording. 

The sound occurs hot or cold however does seem to reduce when revs increased offload.

I'd love a quick and easy fix that would get me to tomorrows track day at Knockhill !

SM25T
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Manifold to head nuts tight ? ... tight, not gorilla thread-stripping tight.
RogerB
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I have now recorded and posted U tube clip however I can't seem to paste the link in here.I have titled it as 'Caterham engine unusual sound'

The lower pitch and lower frequency sound is normal and clears when clutch depressed.

So I have now checked:-

Starter motor - Run Engine with motor off.

Alternator - Run engine with drive belt slack.

Exhaust manifold - For tightness, leaks, contact with chassis/Body. (I once had a metallic sound on my Griffith 500 which had a blown manifold gasket - different to this sound though)

Cam belt cover off - All tight and belt in good order.

Listened with long screwdriver - Can only hear the usual sound of tappets (By the way I like the photo of the Marine Engineer, especially as I'm a retired Marine Engineer, although we never wore Steaming Bonnets in my time)

Spark plugs - Looked good apart from a little sootie with pale brown tips (Running  a little rich I guess)

Distributor - So far unable to loosen one of the distributor cap screws as the column on top of the bell housing dry sump tank restricts access with my screwdriver.

RogerB
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Oh I also forgot to mention:

Mechanical tappets.

No loss of power or other abnormal running.

However and this may be a red herring - During my Croft track day I tried to accelerate in 2nd out of the hair pin and the engine just temporarily died. I thought I was going to have to coast onto the grass until I restarted, checked for oil pressure and temp which where fine so continued with no re-occurrence. It was at the end of this day that we noticed the sound which can't be heard when on the road. I also drove the 250 miles home to Scotland with no other abnormalities.

revilla
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SV VVC 170 - 170.4 bhp @ 7100 rpm - 142.4 ft.lb. @ 4900 rpm

revilla
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Ooh err that doesn't sound healthy! I know that's not a great help but I'm not quite sure what to suggest. It sounds a lot harder and more mechanical than I imagined. I'd be tempted to wonder about a broken valve spring or something, but I would imagine you would notice that running.

SV VVC 170 - 170.4 bhp @ 7100 rpm - 142.4 ft.lb. @ 4900 rpm

revilla
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If it has solid lifters, does it have double valve springs by any chance? If so I could possibly imagine one spring of a pair being broken and it still running reasonably well, especially at low RPM, on the other spring. Just a stab in the dark really ... hopefully somebody will have heard that before and will be along to tell us shortly, I'm as intrigued to know as you are now!

SV VVC 170 - 170.4 bhp @ 7100 rpm - 142.4 ft.lb. @ 4900 rpm

Wrightpayne
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The only noise I’ve had from that vicinity is one of the primaries rattling in the collector.

That sounds a hole different level of mechanical noise. Me thinks a strip down is needed.

Ian 

TomB
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It’s very quick and easy to take the cam cover off. You could turn the the engine over my hand and see if there is anything remiss there? 

RogerB
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Ok thanks guys and that was also my line of thought. I don't suppose its possible to change a valve spring without removing the cyl head. I once recall a Cummins Service Engineer changing a valve spring on a K19 generator engine without lifting the head.

It could well be double valve springs as its a high speck Steve Parker racing engine kicking out 190 BHP at 7200 RPM.. Unfortunately Steve Parker had passed away before I bought the car and I don't have any build spec for it.

I will have to start reading my new K Series Engine book in ernest. Also Toms suggestion of turning over by hand with cam cover off sounds good.

So that's me off to Knockhill with my pal as a passenger. The newly formed Leven Sports car Club have been running these all summer as a marketing event. Hopefully I'll get some hot laps in an Aston Martin or Zenos.. 

Then back to work on mine.