"unfamiliar jingle in jem's pocket"

The wording of this quote is slightly ironic and is a perfect example of Harper Lee's clever interchanging from Scout's young childish point of view to that of an adult with a more cynical, sarcastic look on the world. 
It is also a subtle reference to the economic setting of America at the time. The Finch family are a standard middle class family but at the time the book was set America was going through a depression and money was tough to get.