I have a 99 Ford Windstar that I just bought from a friend. We had to replace the starter before driving it bc it sat for a long time. The squealing sound happens as I drive and does stop the more I drive the vehicle. What could this be? Does the serpentine belt needs to be replaced or can I just adjust it?
You could start by changing the serpentine belt. Your Windstar has a self adjusting tensioner, so it could be this also, but there's no way to adjust it independently of changing it. You can check to see if it's doing it's job by running the engine and seeing if the belt has a lot of flop (not sure how to describe this) when you rev then quickly release the throttle. When the engine spins down after a quick "blip" of the throttle, you shouldn't see much in the way of the belt vibrating. Where to look for this is the longest stretch of the belt between two pulleys, usually just after the crankshaft pulley (large pulley at the bottom). Most engines rotate in a clockwise motion. The crankshaft pulley is where the power comes from, so the slack to the backside (before the crankshaft pulley) is most likely going to be tight all the time. It's after the crankshaft pulley where you're going to see any type of slop occur in the belt. If the tensioner spring cannot keep up with the revs of the engine, it will allow the belt to bobble and it could lose traction, causing the squealing issues you talk about (yes, this is a long winded answer).
It could also be that the battery just needs some TLC (recharging). If it has sat for a while, it could have been discharged quite a bit and still needs some recharging. Right after you start the vehicle, the battery has done it's job, but needs to replace the energy it used to get the engine started. A combination of weakened battery and tired belt could easily account for this. You can check your battery by putting a multimeter onto it and checking the voltage without the engine running. If it's in the 12.5-13.2vdc range, it should be in good shape. If it's below this, try putting it on a battery charger overnight on the lowest Amperage setting there is (2A is good for this).
Really, I think the answer here is the belt. Once you get it changed you'll probably find the squealing will have disappeared.
It really sounds like a wheel bearing. The noise would get louder the faster you drive up to the point where you can't hear it anymore because it is too high pitched.
If the squealing doesn't happen while you are stopped with the engine running, then the issue isn't the belt or tensioner. If the problem were either of those, it would be constant, moving or not.
You want to check this before driving at speed, because, you could really mess up the vehicle if the wrong thing breaks. Breaking it while moving always costs more. For instance a seized wheel bearing can cause parts to be bent which would ordinarily be fine.
The safest way to check where the noise is is coming from is to have someone walk or bicycle along side you while you drive slowly on a non-main street. Let them know you plan on steering to the left and right to check if the noise changes when turning the wheel. They should be able to tell you where the noise is coming from. It is more difficult to tell from inside even if you have all the windows down, though you may be able to tell which side that way.
Whether or not you can replace it at home easily depends on if the wheel bearing is pressed into the control assembly or rear hub. This is a part that's probably not dealer only.