I too at first thought figured this was a hoax story...but it wasn't on Snopes, and when I went to the periodic table and compared gold, tungsten and lead I too came to the conclusion that tungsten might have some advantages in faking gold bars.  I bet the drill bits complained a bit when they hit the tungsten however.

Since Ernie has decided to justly give this a "Publisher's Recommended Reading" label it is important to credit the original source on FP: Phennommennonn Glp., who's incredible story led to this one.

http://www.freedomsphoenix.com/News/061976-2009-11-26-us-govt-manufactured-fake-gold.htm

Now it sounded like a hoax.  But while nothing major has turned up, it turns out tungsten cores would be a very good way to fake gold bars.  If it is a hoax, it is a new one and a good one.

If it is not a hoax then the question arises, why and who had them made?  Certainly someone in charge of the gold in Ft. Knox.  Who could procure at taxpayer expenditure the secret manufacture of these bars to specification.  Who could have them brought into the hardest place to bring fake gold bars into without suspicion -- meaning in regular shipping containers in regular shipping trucks with regular escort and regular paperwork.  Presumably irregular location for pickup.

Motive -- to sell to the Chinese?  Risky.  To be used for show for the fawning media cameras in case any taxpayer wanted to "see" the US government gold stores for themselves -- sort of a Potemkin vault.  Not risky.  While the actual gold bars went ... ahhhhhh, where?   And some idiot didn't know the fake bars were fake and loaded them up for shipment.  I wonder for how many years the US military has been guarding fake bars of gold?  A lot I'd bet.

There are a couple things that make this more difficult than it seems.

First, plating tungsten with gold:   anything gold-plated always has a different look and appearance than actual gold itself. To those who aren't around gold a lot, they may not notice the difference, but it is obvious to those who spend a lot of time with the metal, as those working at exchanges and depositories do. So plating would not work.

Hollowing-out a gold bar, filling with molten tungsten:   the problem with this technique is that tungsten has a higher melting point than gold. The hollow gold shell would melt once the molten tungsten was poured into it.

However, the only technique that appears plausible (I can't prove it, because I don't have that much gold laying around to try this myself) might be to cast a bar AROUND a tungsten core. Conceivably, this would work by starting with a gold "plank" or bottom piece of the box that will soon surround the tungsten core. Setting the core bar centered on the plank, molten gold would be poured in, encasing the blank. This time, because molten gold is still below the melting point of tungsten, there is no deformity. Depending on the thickness of the box walls encasing the tungsten blank, no physical testing of the bar would reveal that the innards are other than gold. Only significant intrusion into the body of the bar would reveal the fraud. Some sort of X-ray or electronic density differentiation detection test would have to be devised that would be capable of "seeing" through the bar in order to test a bar without physically assaulting it.