raid 1 fails twice

I have a 2.4 linux raid 1 on an ASUS mother board. One of the drives
failed yesterday and a couple of hours ago I replaced it with a spare I had
on the shelf. Now, when the re-sync should be complete I find the new
drive also failed and making noise that indicates the controller thrashing
the drive arm around (clink, clink, clink). The controllers are integrated
into the mbrd so I would not expect them to fail without other problems
manifesting themselves. Anyone care to take a guess at what's going on,
I'm stumped? In all other respects the system seems to perform normally.
It is a production web server on-line 24x7

2 20gig WD205AA
SIS 5518 chipset

Michael