Strange Engine (?) noise

Heading down a rabbit hole, thought I’d throw this out just before I jump…

A quick search on this topic turns up many problems and many solutions, not sure which one will be mine!

My 56 140 OTS has started exhibiting a funny noise… usually when loafing along, with engine warm, sometimes a gentle rattle, sometimes evolving into a louder more violent thwacking, after a minute or so seems to subside on its own, does not respond to plus or minus acceleration, ie does not get louder & faster if I accelerate when the noise happens, and vise versa does not get quiter and slower if I jump off the gas.

A short history on the car may lead to clues?

I assumed ownership (custodian-ship?) of the car about 3 years ago from my dad who bought it @ 1982. Overall condition at the time was questionable as that was an era when these cars were not appreciated and service tended to be the quickest and cheapest route back to the road… as illustrated by the C Type head tossed aside for a 120 head when repairs where needed many years ago!

When I adopted the car dad had been driving it as a sunny day car, but in rapidly diminishing amounts as at 90 years of age he found it was getting harder to get into much less drive! and his usually very high standard of self-service was diminishing also as he found it equally hard to get on and off the ground to fix things! (He currently still maintains his 60 Alvis and 57 Bentley… he’s no slouch!)

Over the last couple of years I have given it moderately thorough mechanical inspection and repairs where needed to make it road-worthy…
New front sway bar bushings to replace the nonexistent ones
New water pump, hoses, thermostat, flush rad
Remove , clean and inspect distributor, new points, condenser, cap
Rebuilt carbs including throttle shaft bushings as I could not get idle down below 1000 rpm.
Changed engine oil, filter, spark plugs and rear axle oil
New (to me) 54 spoke wheels, new tires and tubes, broken spokes replaced, tire and tubes self installed
Lubed all chassis points
Dismantled, cleaned and inspected all brakes: no signs of leaks, wear and dust accumulation seemed even amongst all wheels so back together with nothing more than the cleaning and adjustments.
Cleaned under chassis, engine, transmission, OD, rear axle, inspect and tighten all nuts and bolts to minimize leaks… OD leaks significantly at actuator arm, nothing else leaks more than expected.

Proceeded to adjust points gap (dwell), timing, carb settings… started having the "noises ". Noises were not present in similar drives before all the mentioned work…

Thinking this is some form of valve rattle (although in my experience that is usually most evident under hard acceleration which is not the case here) I went back through everything…

Pulled front wheels looking for loose or broken spokes, pulled front brake drums looking for improperly installed adjusters or anything that could have come loose…
Re-did Gap, confirmed with Dwell.
Re-did timing, confirmed function of mechanical and vacuum advance with additional marks I put on flywheel.
Drained and refilled gas tank with fresh 94 octane gas
Inspected all front body work, panels to ensure nothing was rattling

Since the second tune up the noise seems to have reduced in frequency and severity, or maybe I’m used to it and not as freaked out!? But it needs to be resolved… generally car is running smoothly, idling well at @ 600 rpm when warm, pulling strongly up hills, plugs are nice and light brown when inspected.

So the plan forward…

Pull valve covers, look for anything amiss… note tach is not reading past @ 2500 rpm but this has not changed in years, will inspect tach drive but unlikely to provide the "energy " needed to create such a loud and violent noise.

If nothing is evident, pull the head… looking for broken valve spring?, bent valve? Maybe timing chain tensioner?

Trying to think of what I have touched in my inspections and services that this noise could be related to… if it is a problem I have created then it’s not in the valve train or engine…

And having typed all this out I feel much more focused so thank you for listening!

Thoughts, related experiences, tips all appreciated!

When cam box covers are off, ascertain the tightness of the upper chain, the condition of the upper chain guides, and see if you can observe any slack in the lower chain.

Also inspect the tappet guides, and see if any have migrated up, out of their bores.

First try a mechanic’s stethoscope, or the low budget version is a long screwdriver in the ear, see if you can locate the noise in the engine.
If nothing is obvious, I would start looking for loose sheet metal, loose fan blades, exhaust hangers, etc.

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Thanks Paul, that is where I am headed!

Hi Rob, I had hoped to do this, but the noise happens intermittently and I haven’t been able to have it happen while in the garage. I did pull over promptly once when it was rattling, but by the time I got the hood up the noise was gone… I have gone through all the fan, sheet metal options a couple of times now, at first thought it might be something interfering with the fan…

The stethoscope still might detect something.
I would try eliminating things. Disconnect the tach cable at the camshaft and speedo cable at the gearbox. Run it up to a good speed, put it in neutral and shut off the ignition, and just coast, see what you hear while rolling with the engine off.
Remove the fan belt and run it without the water pump and generator turning and listen with the stethoscope. You can do that for a minute or two.
But I keep coming back to timing chain.

That would be my vote. The tappets will generate a ticking noise that varies with engine speed if something is amiss. If the chain guides are failing, it could account for the intermittent nature of the noise as the position of the guides and the chain changes. You could put an endoscope down past the upper chain and sprockets, but my guess is the damage would have to be significant to be seen.

Dave, does your engine have the coil spring lower chain tensioner or hydraulic tensioner?

Doubtful the upper chain guides are implicated if they’re just worn, though possibly one has broken off. When the aftermarket guide on the left side of my 4.2 rebuild broke off there was an intermittent rattle as the thing bounced around. The rattle of course disappeared when I fished the piece out and the engine sounded normal again, with only the right side guide still in place. (So, if the engine sounds normal with only one guide in place, it stands to reason that wear alone will not be manifested by a rattle.) I contacted the parts supplier and was told that they’ve seen original, unrebuilt XK engines with broken guides, so it happens with OEM upper chain guides too. Easily enough to check out after removing the cam covers. Wear or bonding failure in the lower guide may be another story, since unlike the upper chain the lower chain’s tension is variable and the long run up to the intermediate sprocket may be subject to excessive lash, though if the rattle is from the lower chain I would sooner suspect a tensioner issue.

Don’t know! It’s engine # G3892-8S if that helps!

Ok most likely you have the coil spring type.
The coil spring is the yellow arrow.
It pulls the blade (red arrow) which pushes against the chain to hold it tight.

I had my coil spring break once and it made a rattly noise which could be heard down low with the stethoscope.

…and that is almost always a result of a loose upper chain.

As part of periodic maintenance on customers’ cars, I checked/tightened the upper chain every 10,000 miles.