Its running the daily and weekly maintenance scripts.  As root, do 
crontab -e
or just look in /var/cron/tabs

and you'll see the entries which run daily, weekly and monthly scripts.

Anyway, the short answer is its normal for a few minutes of lots of disk
activity and machine slow down at that time of day.

-Dan


On 16 Jun 1999 shigeki@mediawars.ne.jp wrote:

> Hi,
> 
> This is a strange story. I'm running 1.4C on LC520 with external 1 GB 
> HDD. When the clock points to 3:15AM (JST), external HDD (it's running 
> NetBSD/mac68k at that time) starts to make a loud noise and system slow
> down. It's daily happening.