A couple of weeks ago, I opened up my computer to clean out some dust, I took off the fan (not CPU fan, the exhaust fan) and got all the dust out of it fine.

For weeks it has been silent, but now it's making lots of loud noises. The noises aren't related to air rushing out of the computer; it doesn't sound like a hair dryer.
But instead the fan sounds like it has a gas-powered engine.

This happens intermittently, sometimes the fan is silent.

I took it off and re-screwed the fan in tight just in case it was vibrating on the case, but to no avail.
I've looked around the internet for solutions, but all are referring to CPU fans constantly spinning too fast.

Also, when I look at the fan, it looks like it's not really spinning as fast as it should.

Can anyone tell me what I can do to make my fan stop screaming at me?

Buy a new one. Perhaps it's hitting a cable or it's just dying. I would monitor the temperature of your CPU and make sure it's not getting too hot, but in the meantime order a new fan.