Hacked? How can I tell what process is sending packets from a particular port (udp/55613)?

[boink]

Dear FreeBSD,

I see outbound packets from udp/55613, one every 5 seconds, to a
single non-routable (10....) IP, with destination port increasing by 1
with each packet, with expected ICMP Destination net unreachables from
an upstream router.

AFAIK, there's no reason for this and I don't like it - how can I tell
which process is sending the packets?

With thanks in advance,
boink