I hear the same strange noise on poweroff (2.6.20 vanilla). This is what I
get on my system with ATA_DEBUG and ATA_VERBOSE_DEBUG both defined. The
system will *not* poweroff with "halt -f" so there was no noise!
md: stopping all md devices.
Synchronizing SCSI cache for disk sda:
ata_scsi_dump_cdb: CDB (1:0,0,0) 35 00 00 00 00 00 00 00 00
ata1: ata_dev_select: ENTER, ata1: device 0, wait 1
ata_tf_load_pio: device 0xA0
ata_exec_command_pio: ata1: cmd 0xE7
ata_host_intr: ata1: protocol 1 task_state 2
ata_hsm_move: ata1: protocol 1 task_state 2 (dev_stat 0x50)
ata_hsm_move: ata1: dev 0 command complete, drv_stat 0x50
If I do a sysrq-o right after this, the above message repeate once more and
the system turns off w/o the disturbing noise.
For the next messages, I booted the system completely (no init=/bin/sh) and
did a "poweroff" at the console prompt after the boot was completed. This
time the noise was there.