Weird GEOM errors

[hunter morgan]

Hi.  ive had a freebsd7 box running zfs on 5 500gb hard drives for months
upon months now with mostly good stability but every couple of minutes in my
logs i get errors such as:
kernel: GEOM: ad2: corrupt or invalid GPT detected.
kernel: GEOM: ad2: GPT rejected -- may not be recoverable.

on 2 disks, ad8 and ad2
any ideas why and how to fix? mostly just annoying. thanks.