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??