carl wrote:
> Hi,
>
> Can I bring another question to this forum ? I looked at the archives, but
> found nothing similar for this noise pattern or situation. And I wonder what
> a mechanic would say, given lash values are correct for most of the dealers.

Is the valve lash correctly set? Was it set to the correct gap with
the engine at the proper temperature?

>
> My car has had valves tightened a bit too much, now they're at correct valve
> lash. For a while, I hear too much noise coming from the cover. Valve lash
> increases with temperature ( is it always so ? ) and seems lower in
> frequency and much less noticeable when warm

How do you know the valves were tightened too much.

>
> It is loudest on a cold engine, and in the afternoon, with engine still
> warm, is never that loud again. It also becomes very high in frequency when
> cold when the throttle is released, after driving in low gear and low load,
> the clatter increases just after release in volume and frequency, and
> diminishes after about a second ...

A rattling or rapping sound can come from a lot of sources besides
valves. An exhaust leak can make a rattly sound when cold, as can
loose wristpins, main and connecting rod bearings as well as valves
that are not correctly gapped.
It is all but impossible to correctly diagnose the cause of noise from
a written description on this or any other forum. I would suggest that
you have an experienced mechanic disgnose the problem, preferably one
familiar with whatever make of car you have.

>
> Does the noise pattern, like after throttle release, point to something
> worse ( low- high end or main bearing ) ? I thought they would make heavier
> noise. There could have been oil issues in the past.

What is an oil issue. Do you mean the engine was run without oil, or
the oil was never changed?

> Also, timing could have
> been too early 4 degrees for a long time, or late 4 degrees for 1000 miles
> now, giving more than normal bearing load ?

How did you determine the ignition timing had been reset over such a
wide range.

> Now this valve train has a OHC, two rocker shafts and 12 valve rockers with
> 4 exhaust ones. The rockers are cast aluminium, with iron or steel adjuster
> screws. I imagine it is not such a big deal to install new rockers and
> shafts, and even a camshaft, and adjust valve lash again. But from where can
> this noise come when valve lash is correct ? The car has 35 k miles and oil
> was changed just at 33k5 miles. Is there something with aluminium rockers
> and steel adjustment screws, which should not be overtightened ( they
> tighten even further when hot ) ? They could have been.

Don't start adjusting valves and replacing rockers before you know what
the problem is. I would strongly suggest that a mechanic with the
proper tools and training take a look at the car.