You should be able to do this sort of thing by looking up LFOs in the various tutorials / manuals (eg: http://en.flossmanuals.net/pure-data/)

In fact, attached is a patch with a very simple LFO assigned to the pitch of an oscillator. It should be quite straight forward to adapt this to an FM model. Just think about which part (LFO) of the FM model you want to use for the modulation.