MacPro 1,1 clicking sound (not HD)

10-11  Source: Network gathering  Views:2 

I have an odd problem with my MacPro 1,1 that I hope someone has an idea about.  It makes a clicking noise when rebooting -- and only rebooting -- and then the reboot stalls at a grey screen until I power it off and back on.
I thought, logically, that it was a hard drive problem.  I pulled all the drives, put in a new one and reinstalled Lion from an external drive
However, the clicking was still there.  So I tried pulling the new drive and rebooting with only the external Firewire drive connected (it has an SSD in it, so no moving parts) and it still made a clicking sound without any drives attached
What else in a MacPro could cause the clicking sound and a reboot to hang?
Thank you for the replies.
I have two copies of the HD on external FW drives, and I keep ALL my personal files on external USB drives, so it is not a problem if the HD fails.
Interestingly, though, the problem cannot really be called intermittent, because when I start the computer from a complete off, it is guaranteed that it will click repeatedly and the rainbow will spin at the same time for a few seconds...
and if I then restart the computer, it is "guaranteed" (based on daily use during the past several months) that there will be no clicking or rainbows whatsoever.
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