I have a SunOS 5.8 server. I suspect there may be errors with one of the physical disks (based on someone reporting hearing a "clicking sound" during the night shift, however I do not hear anything this morning).

Which log would show this?

Thank you.

It matters if syslogd is running or not and how it's configured.

Look for the process syslogd - if it's running, look at how it's configured by looking at /etc/syslog.conf.

Any errors could be logged to console (which may or may not have a log - it matters how it's set up at your site), or /var/adm/messages. If syslog.conf was changed from default, it could go to whatever file is listed (or not get logged at all).

Dmesg command may also have the errors (even though it's obsoleted by syslogd)

If the box has not rebooted since the possible errors, try:
iostat -En