Did you have them look at the engine mount?

Always very difficult to diagnose things like this from a distance.
Can you tell which side it comes from? A passenger in the car with you might
help pinpoint its position.
Again, is it a clang, or a knock? Favourite on these models is the anti-roll
bar bushes, and drop links. A worn drop link will make quite a sharp noise,
but the anti-roll bar bushed make a more dull thud.
Also a badly worn lower ball joint can be noisy.
Are you sure you haven't just got a loose exhaust somewhere, as that would
match the sound.

I would go for worn p bushes.  i get a lot of these to do as there a
crap design.  And if you lower the car they wear out even quicker.

Also another bad pint which produces a knocking is the rear bushes which
cause a knock when you go over speeds bumps.

Sean