What process runs every 30 minutes

The only entries in /var/log/messages are from dnsmasq-dhcp.

The /var/log/cron shows jobs running every hour, not every 30 minutes. Most of them run at hh:01 apart from a clamav job that runs at hh:45. The process that spins up my disks runs at hh:22 and hh:52.

Cheers.