Mysterious disk activity

[Pigeon <jah.pigeon@ukonline.co.uk>]

Hi,

I get these occasional very long bursts of disk activity, usually but
not always within an hour or so of booting up, during which the HD LED is
on continuously and the machine is very slow to respond. There were
cron jobs running global finds, which I knocked out; this helped, but
didn't stop it entirely. To make it more mysterious, ps ax during such
a burst shows nothing untoward:

  PID TTY      STAT   TIME COMMAND
    1 ?        S      0:09 init
    2 ?        SW     0:00 [keventd]
    3 ?        SWN    0:00 [ksoftirqd_CPU0]
    4 ?        SW     0:01 [kswapd]
    5 ?        SW     0:00 [bdflush]
    6 ?        DW     0:00 [kupdated]
    7 ?        SW     0:00 [scsi_eh_0]
   79 ?        SW<    0:00 [loop0]
   80 ?        SW<    0:00 [loop1]
   81 ?        SW<    0:00 [loop2]
   82 ?        SW<    0:00 [loop3]
   83 ?        SW<    0:00 [loop4]
   84 ?        SW<    0:00 [loop5]
   85 ?        SW<    0:00 [loop6]
  112 ?        S      0:00 /sbin/portmap
  164 ?        S      0:00 /sbin/syslogd
  167 ?        S      0:00 /sbin/klogd
  172 ?        S      0:00 /sbin/rpc.statd
  182 ?        S      0:00 /usr/sbin/inetd
  193 ?        S      0:00 /usr/sbin/rpc.nfsd
  195 ?        S      0:00 /usr/sbin/rpc.mountd
  201 ?        S      0:00 /usr/sbin/atd
  204 ?        S      0:00 /usr/sbin/cron
  215 tty1     S      0:00 -bash
  216 tty2     S      0:00 -bash
  217 tty3     S      0:00 -bash
  218 tty4     S      0:00 /sbin/getty 38400 tty4
  219 tty5     S      0:00 /sbin/getty 38400 tty5
  220 tty6     S      0:00 /sbin/getty 38400 tty6
  454 tty3     S      0:00 mutt
  619 tty2     R      0:00 ps ax

top shows CPU usage of a few percent.

Any ideas what's causing this?

Pigeon