This could be from a variety of things.
The first is from your HDD. Many times,
you can hear odd clicks at random times
and can be seen as 'normal' (ie. impending
premature HDD wear) If your HDD is located
in the upper right corner then that would be
where the source of the sound is comming from.
However, it can also come from your speakers.
If set too high, there might be a background
battery save feature that causes the clicking
sound when there is no sound for x amount of time.
Then theres' always the possibility for something
else.

My bet would be on the HDD.

Don't forget, new laptops may or may not need
a 'break in' period. Call Dell support and ask
if they have anything like this in their KB and
FAQ's