The smell or halitosis around the dental crown or a tooth structure is due to bacterial leakage and production of sulfide compounds as the result of bacterial breakdown. The crown fit needs to be checked, the gum health needs to be checked and unfortunately possibility of decays exists!
An expert dentist can diagnose this, by taking x-rays and does his clinical exam!
In certain cases the crown is over a root canal tooth, in which there is possibility that the root canal is gone bad and there is bacterial leakage and infection under the root canal, in which unfortunately you need to redo the root canal again, or do the root canal and the crown over again if there is decay involved also.