One of you will know what this is...

[rocco16 at sbcglobal.net (Larry Fry)]

Driving to work yesterday morning, I notice a 'tick-tock' coming from the
dash.
 I time it to see if it's random or consistent...yep, every five seconds;
tick-tock.
 As the miles go by and my concerns mount, I realize that, in conjunction
with the tick-tock, there is a soft buzzing from the rear of the car.
  Every time it ticks, the buzz commences... with the 'tock', the buzzing
ceases.

Logic tells me that the tick-tock is the fuel pump relay and the buzz is one
of the fuel pumps (the one in the tank?), but why is it doing this?
The car ran fine....and the trip home from work was without this phenomena!
I need your opinions.  Is a sudden and complete shut-down immenent?  Or
should I happily ignore it until it becomes constant?  I need to fix
whatever is wrong...so, WHAT COULD BE CAUSING THIS?
Thanks.
Larry
sandiego16v