[Chromenut]

Hard to say, you've not really given enough info for a good opinion. Please tell us the year and model (as in is it an NA or turbo), and the mileage, and the area you live in so that we can understand the climate you're in, and last any work you've had done to it. That would help tremendously in forming an opinion. I can say that the tensioners do have a history of squealing. If it were mine, or if mine starts up, I plan on replacing both the belt and the tensioner just to be safe.

If the tensioner is where the squeal is coming from, they tend to really heat up and that adds to the wear on the belt, as a lot of that heat gets transferred to it when running. This is why I'd do both. You can always try the old bar of soap trick to see if it's the belt. Take a bar of bath soap, nothing with any grit to it, just plane jane soap, and while the engine is running, hold it against the back of the belt and allow it to pick up some of the soap. Be very careful as where you're working there's a lot of moving parts and hot items. If you get a good coating on the belt and it still squeals, I'd say maybe the tensioner.

Only suggestions from your old uncle Chrome. I take no responsibility for the outcome of any of my suggestions as it's possible that I was embibing in alcoholic beverages when typing...