Powerbook G4 Hardrive Gone Missing


My Powerbook G4 froze. I powered off and since then can't start up. What I've tried:

reset PRU
reser PRAM
Started up from install disk, went to run disk utility, but no harddrive shown!!!

tried to boot up with ext. off.
tried to rebuild desktop

put in apple hardware test disk. booted up from that and ran extended test. everything passed.

now completely stumped. am running off of external harddrive with os X installed and computer is making loud clicking noise but giving me no joy in locating hard drive.

to make matters (much) worse, somehow or another I thought I had everything backed up on said external harddrive but nothing is on there except the really useless stuff that I don't know why I saved anyway.

please help.