Detroit 60 Fan Noise

Most likely it's a belt tensioner going bad. Definitely a bearing in one of the pullies though. Find it or risk it locking up and snapping your belt. Or even melting the pully and setting your truck on fire.

 

Loose belt

 

or your alternator is about to die.

 

The check light coming on may be due to ecm putting truck in derate mode at 25% less power because ecm is trying to protect engine.

You may also want to check your thermostats being there are 2 of them, one may be bad.
I agree with belt tensioner could be seized or on it's way.

Watch the engine at various idle speeds to observe the belt .
Than remove belt and and check all pulleys to see they are free.
Also your alternator may be getting tired. And if you have the old large style those are known to lock up more often than the new compact style alternators.

What year truck or engine??

 

After running for a while stop and put your hand on the alternator, If it is very hot you’ll know it bad. If the Check eng light is coming on what is the code?

The ac can turn on the fan, check the compressor for signs of slipping belt.
The ecm turns the fan on for water temp, my ac doesn’t go thru ecm.
The loss of power could be wiring harness causing your problem.

 

It could even be a cell in the battery giving up. When the weather changes to this cold we getting slammed with now many of those little problems become big quick. Start with easiest and cheapest, like battery connections clean and dry?

 

Clutch is bad in the fan . That or you losing air to engage the clutch. If fan isnt spining fast enough the sensor might be sending check engine light. Buy new clutch simple as that