Answer to "Mods for tiger?"On the topic of non-native OS support. There is a tool out there that I remember reading about that will temporarily change the "processor speed" to make it compatible with leopard. I put that in quotes because it doesn't actually change it, but rather, it changes the cpu report in the NVRAM(? I believe), so that you may fool the Leopard OS disk into installing onto a Mac that runs slower than the 867mhz Leopard minimum. I cannot remember where I saw this tool, but I would recommend googling it and checking the Hackintosh forums. Beyond that, max out the RAM, inquire about a HD upgrade, keep your login items to a minimum and clean the computer regularly.
Answer to "hard drive - noise and clicking"The 7200 RPM drives are known for having loud noises when installed in laptops. That is, as I understand it, the reason apple does not install them from the factory. If the OS is pausing and you're hearing clicking, I would assume that there is a problem with the HD. Some HD's are just loud though. Best way to test it would be to mount another HD inside and clone (http://www.bombich.com/index.html) your system to see if the sounds and pausing OS problem continues.
Answer to "What type of OS is in my PowerBook"It depends when you bought it. Some PBG4's came with Panther (OS X 10.3), most came with Tiger (OS X 10.4), although it is capable of running Leopard (OS X 10.5). To find out which one you are running, see Tyler Bly's Answer. (Go to Apple Logo in upper left hand corner and select About this Mac from the drop menu).