Powermac 7600/OS9

Did you install an "active" terminator on the unused port (DB-25 or Centronics-50) on the back of the external case? For identification as to type, terminators are often marked "ACTIVE" or "PASSIVE." For system stability, you'll have better luck with the "active" type. As for the loud noise, heads that are worn down until the tip of the arm is scraping the platter's surface can sound like an electric razor.

first, the display.

You display may have accidentally become set to a too-high resolution, and be showing only the center of the screen. An older VGA display may not have internal 'capabilities' it can report to the Mac for auto resolution-setting.

Safe Mode does a number of different things. On older Intel Macs, hold shift at startup, but have your userid and password at the ready.

A parade of unusual things happens.

• Your Mac loads just enough of the kernel to do a disk check. Then it proceeds to do a disk check. This can take an extra about five minutes.

• your userid and password are required, even if you normally auto-login. So have them handy.

• Your Mac adds ONLY a minimal set of Apple-Only extensions, Not including graphics acceleration extensions. Screen updates will therefore be wonky and slow, but it ultimately should be correct.

• Your Mac assumes defaults for as many settings as possible, including screen resolution. This is the key for re-setting the screen, but there is a little more to it: Resolution is likely to be lower and settings ordinary. Use this as a starting point to customize settings to your liking.

Any changes you make in Safe Mode will "stick" in regular mode after you restart.

• after restart in normal mode, your Mac will take slightly longer to start up [once] because it rebuilds some system caches.

"Works in Safe mode, fails in regular mode" implies "It's something you added".

How to use Safe Mode on your Mac

Use safe mode on your Mac - Apple Support

The motor in large multi-platter SCSI drive can output a surprisingly loud groaning sound when it fails to spin the platters. It can be almost musical. Solutions to get it un-stuck are to get the drive slightly warmer than room temperature before trying to spin it up. If that does not change anything, try placing it in the freezer for a while. Results vary wildly.

The other case:

I had a drive that crashed its heads into the platters. The sound was loud an extremely unpleasant. It reminded me of the sound of the steel trolly-car wheels against the rails on the Boylston Street turn in Boston, the tightest turn of any light-rail system in the US.

Interestingly for me, it was a removable cartridge drive, and I was free to look inside it later. The heads had literally plowed a trench through the brown oxide and exposed the shiny metal substrate of the platter underneath. It made a mess, and sprayed oxide everywhere inside. That drive was completely ruined, with no hope of recovery.

I disagree.

When SCSI drives and their cables disconnect, they ruin the cable Termination, which cause every drive on the cable to stop working completely, and sometimes causes your Mac to hang as well. Data error or 'creeping crud' are not caused by that.

it is really difficult for anything just sitting on a connected drive to do any damage to your Mac.

The most likely cause of the troubles you are describing is something else entirely.

was that sound horrible?