Bathroom exhaust fan always running?
I installed a fan in our bathroom to improve air circulation in the house. It is intended to be run all the time at a low setting. We have a motion sensor module in ours that will increase the fan speed (and thus the CFM) for a period of time (settable). I kept the switch on ours, but you're not generally supposed to. Instead, it is common to swap the switch with an outlet, like it appears was done in your case.

A house is intended to have a continuous exchange of air with proper ventilation. If you do not have that, your indoor air will quality will suffer... more CO2 and humidity, leading to mold growth. If mold is detected in a house, a common recommendation is to increase air exchange by installing a continuously-running fan.

If your fan is running all the time at a high rate, it may be possible to install a motion-sensing module. Google the fan's model number to get a manual to find the right part numbers to buy.

If you think your air quality is fine, and you want to make the fan switchable again, you should be able to easily swap the outlet back out for a switch. Generally speaking, you'll want to remove the outlet (with it's pig-tailed wire(s)) and then find the wires that go to the fan... there will be two coming into the box in the same spot which, when disconnected will leave a working light switch, but the fan will be off. One of these two wires will go to one side of the new switch and the other wire will go to one of the existing capped bundles... then you'll need to add the pigtail from the other side of the switch to the other color capped bundle.

Take this all with a grain of salt... I'm not an electrician. Be sure to turn off your breaker when you are doing the changes. If you're not sure which wires are which, it's best to make one change at a time so you can reverse it as needed. I always take a picture before I make any changes. Writing it out on paper sometimes helps me get my thinking straight.

Finally, if you just want to split the difference and be done with it, you could consider just turning the CFM dial down until it is quieter but still moving decent air.