NEW YORK (CNN/Money) - If you ever want to knock a market down, come up with a rumor that's A) plausible and B) impossible to substantiate.

If you wade into the bond pits lately, you'll find plenty examples. The most effective yet is probably the one that says Asian central banks are going to shed agency securities -- the debt of government-sponsored mortgage-finance giants like Fannie Mae and Freddie Mac.

Justin Lahart, senior writer at CNN/Money, talks about the power of rumors in the bond market. Play video (Real or Windows Media)

The chatter got going last week when Bloomberg reported that the European Central Bank was cutting its position in agencies and was recommending that member central banks follow suit. This action had been rumored in the bond market for at least a week, and when Bloomberg substantiated it (at least in the eyes of bond players), agencies fell hard.

But Europe is just a two-bit player in the agency market compared with Asia. For European banks to trim positions was bad enough, but if Asian central banks began shucking their agency debt, it would be a disaster. And not just for agencies but Treasurys as well -- because the agencies are deemed by the bond market "too big to fail," their debt tracks comparable Treasurys closely, offering just a slightly higher yield. Agencies fall, Treasurys fall, too.

Are the Asian banks going to sell? Most bond players think it's quite unlikely. The problem, however, is that they have no way of knowing exactly how unlikely it is.

"I feel pretty comfortable giving the odds on whether somebody like Merrill Lynch is going to sell," said one bond strategist. "But what some bureaucrat in Beijing is going to do -- I have no idea. Is there a 10 percent chance they sell, or a 20 percent chance. So you have this big indeterminate thing weighing over the market."

Such uncertainty tends to draw investors who might otherwise buy away from the market. Leaving only players who want to sell.