Rumors can be either more or less believable based on the information within them, the source, and so on.
For example, if a random person said, "I heard a rumor that the inside of the X-Box 720 is made of the rarest truffles known to man". That would be not a very credible rumor.
Now if a person with a reputation in the game industry on the line who has provided credible information in the past says "I heard rumors the PS4 will include 4 GB of Ram" that rumor would be more believable.
Not all rumors are created equally.