Don't quote me on this while I could be wrong but I believe sr is for scsi emulation needed by the 2.4 kernel. AFAIK 2.6 doesn't need it anymore, although I'm not sure what happens with the noscsi cheatcode. So even if you have no scsi devices they still could get treated as such.

If dmesg is too filled with mysterious messages you can grep from syslog as root:
grep hdc /var/log/syslog |tail
grep CD /var/log/syslog |tail
grep scd /var/log/syslog |tail
grep sr /var/log/syslog |tail