Consider replacing belts again when installing fan clutch. The Fact that they"loosened",you tightened,they "loosened again is from the rubber being burnt away. Just like a burnout/squealing tires=rubber worn off tire. If it was an emergency and you were trying to get car home,i'd put fan clutch on and run them,but replace them when home. Even though the belts "were" new,they'll have no service life as many thousands of miles of rubber has already been worn off them-in the critical area of the pulleys.

Replaced the fan clutch, still have the squeal. It's fine until I hit about 4k RPM after the car warms up, and then it screams like a scalded pig. So I've replaced belts, tensioned them properly, and replaced the fan clutch. Anyone got any other ideas?

Stay on track,you should be able to drive car to duplicate noise then immediately pull over somewhere safe-leave car running. Open hood and grab throttle arm on throttle body or cruise control cable and rev the engine 4k+til you can get the noise to occur.
Will be much easier to pinpoint noise when you're right there versus in the car. Pay close attention to how belts are running,especially when noise occurs.Are they spinning smoothly or do you see one have slack on one side of pulley or whipping around a bit.

For testing purposes,process of elimination,drive til duplicate squeal then pull over and remove fan from clutch assly and toss in back of car and go drive again. See if noise is gone. Fan is not needed above 15 mph,so no concerns about overheating.
If you STILL hear noise pull over,open hood and rev engine as before. If noise still present it will be easier yet to pinpoint without noise of fan. Reading back thru your thread,it is belt/drive/accessory related. You must be overlooking something. Recheck alternator/water pump/air pump for bearing noise or roughness. Check to make sure the four 6mm bolts holding accessory pulleys to main pulley are tight and all pulleys are running true.

That's a good idea. Replacing the fan clutch, belts, and cleaning the pulleys helped, but the squeal is still slightly there at 5k RPM and higher. At least the car is drivable now. The only other thing I can think of off the top of my head is a bad pulley(s) that needs replacing.

Edit - you know, lately my SE has been stalling out after I start it up cold, unless I give it gas long enough to get going. You may be onto something with the air pump.

i have rebuilt a fan clutch on my 280Z, they used fish oil to fill it up from factory, so used fresh synthetic oil to refill it and all is well, before dropping 300 bucks on a new one take it appart clean and refill with fresh oil see if it helps.

I haven't been able to replicate it, and I've wrung the car out pretty good a few times after it's been warmed up (5k RPM +). I'll keep driving it and report back shortly. I'm not exactly happy the noise just disappeared; it's less likely the problem is fixed and more that it's just hiding now.