Hope this helps. Be sure to read the ReadMe first file for information on how to use Incognito. This allows you to use KeyServer, Quota, or some other licence management program on serialized programs such as Photoshop, Director, etc. Fortunately I do not hear it, but everybody else does and often I have to close the door of my room because of it. When I set the monitor darker, the noise is reduced.
Help is very welcome.
I've got an LC630 with system 7.5.3. Disconnecting it - the PowerBook works fine. I suspect it's SCSI related, and that the Zip might complicate matters. Try it without the Zip and also without the Zip driver active. You can also try changing the order of devices on the chain, and perhaps moving the terminator to the middle of the chain.