We have a forced gas heating furnace

After the blower stops, it sounds like a pounding sound from the duct.

I would liken it to a dripping water faucet where the sound comes every 10-20 seconds

Without being able to actually hear it, I'm guessing the sound is that of the metal cooling & contracting.

I agree with Grady. Sounds like expansion and contraction issue. Not a safety or health issue, but annoying none the less. If you can live with it, do so. If you cant, you will need to pinpoint the areas of the ductwork that is causing the noise (may be various areas, and trust me, can be very hard to locate the source). Is this a new issue, or has it always been this way? If a new system, I would suggest contacting the installer and see what they can find. Even if its an old system, you will most likely have to have a pro in to see if it is reasonable to have the noise cured.