I installed a new harmonic balancer a few months back. I don't remember if that is when it started, but maybe the pulley isn't lined up perfectly? The tensioner is still hard to pull down, so I don't think it is that. I have Mac UD pullies I installed over a year ago. WHy do my belts squeak?
Did you recently switch the type of belt (felt vs rubber)?
You might have a slight misalignment and it took some time to glaze the belt. Can you isolate which accessory the noise is coming from. Also make sure it is not a pulley bearing.