Thanks for the suggestions.
I don't think it's any of the two harddrives. The number one reason is that the computer boots up fine and I can read/write from them, no problem. When the click comes wich is pretty loud (makes me jump sometimes) :-) it will reboot most of the times.
I don't think it's a heat-problem cause I've been using my computer this way for 1.5 years, this began recently. The hardware is the same since 1.5 years except for a change from a Voodoo 3 to a Voodoo 4.
I've tried to disconnect the CD-burner to see if it's a case of two many units being connected but it didn't change anything. Besides I think that the Power Tower CPU shall be able to have two harddrives and a CD-burner connected without problems.
If it was a harddrive-problem I should get read/write errors and stuff wich was the case with my IBM harddrive that recently died.
I don't have another PSU to try but I thought that if it's likely to be the problem I could order a new one from Elbox.