Clicking, rattle rom engine

When I drive my Merc and as I gently accelerate to pick up speed obviously
putting a certain amount of load on the engine, with revs to 1000 or 1500,
from the engine compartment I hear a sort of rapid " clicking or a rattle"
this can easily get drowned in the engine noise with higher revs or road
noise. Its frequency does not change i.e. the rapidity stays the same and
doesn't seem dependent on the speed of revs.

Is this normal??

[16VAlex]

How many miles is your car has. That would helps.

Thanks 16VAlex for the reply. Actually it was my friend who was having the problem and I asked him to become a member of this group and post the question. He is a new member now, pathan, and posted the question in a new thread faint clicking/rattle from S500
he has 8500 miles on the car.