The interweb is a wonderful or bewildering place, depending... so I have
been researching what to look for in figuring out what's causing the
clacking noise in my engine. I feel fairly sure that it's at the top
end because the noise seems to slow compared to engine RPM. The noise,
which is hard to describe in text, has a sort of clacking quality with a
bit of a ping to it, sort of like the sound of a poker chip hitting a
table.

But there are a lot of parts in the top end...

So I came across "valve hushers:"

http://www.volvoclub.org.uk/faq/EngineMechanical.html

of which I have never heard before. Could it be as simple as one of
these things giving up the ghost and letting the valve stem clatter
against the cam follower? How likely is this sort of thing?

Repairing this would seem to be be beyond my mechanical skills, but I
think I should be up to pulling the valve cover off and inspecting the
top end of the engine. What would I be looking for (or could I even
evaluate this without pulling the cam and looking under the followers)?

Thanks to links posted by someone else, here's at least a description of
a broken valve spring as suggested by Tony:

http://www.volvoclubofbc.com/technical/springfailure.html

The symptoms don't all match (no loss of power, for example- the car
runs well except for the noise) but I don't know how inevitable those
symptoms would be with a broken spring.

Tim McNamara wrote:

One method you could use is to go pick up a whole head at a salvage yard
and stick it on there. It's the shotgun approach but if it's a top end
problem that will fix it.

Tim McNamara wrote:
They just stop the valve chattering, which is a much less severe noise
than you describe (I think), but its hard to know, maybe its not a real
problem atall. As it says they are usually ineffective in an old
engine, usually flat and hard. Tappet noise is more common in cars with
levers and push rods, but the 240 can get a little chattery with time.
With the bonnet closed it should not be noticeable.

I'd generally agree, the valve would be sitting open slightly and the
engine would run on 3 cyls, but it might just be broken a little bit.

Good research anyway, the more you understand the inside the more
confident you will feel about dissassembly, its how we all started. The
only complicated bit about refitting the cam is the timing belt, and
that is straight forward if you follow the instructions.

--
Tony

Tony wrote:

Look at it this way, it's already broken, the resale value in that
condition is low, what harm is there in attempting to fix it? At the
very least you'll learn something.