Mine is doing the same!!! Although mine was standing in a garage for about 8 months and went started after that there was the same kind of rattle you are describing.
The only way I can explain it is it sounds like one of those old wooden football rattles that you spin round, but not so loud. The rattles are about a second apart on idle but speed up as the revs increase.
Again there is no affect on ride etc, but i think the gear change has always been slightly on the rough side, it has 37k on the clock!