What makes a belt squeal on a 2004 mercury grand marquis

50k isn't new but also isn't that high a milage , how old is the belt ,as age as well as mileage are the factors when replacing belts , chalk dust or french chalk , which are the same thing , thrown onto the belt just as starting should stop the squeal , the squeal is caused by the inner side of belt that runs around the pulleys becomes shiney and cant grip as well as it used to

You need to check the pulley alignment of all the pulleys, they should all be in a perfectly straight line, otherwise the belt will squeal

Is it possible that the belt tensioner is the cause for the squeal or possibly the belt itself needs to be replaced. Although not impossible, the lower pulley usually is not the cause for belt squeal, unless it is damaged.

put belt dressing on belt it will stop squeeling.

check the tension. cheap belts will stretch. if after you retension them (especially the alternator belt) you may need to replace them with BANDO or OEM belts.

Get the serpentine belt inspected or replaced.

To test open hood and while its squealing spray some water on it. If it subsides for a few seconds then you know where the source is.

Could it be the pulley on the AC that's squealing? Use a mechanic's stethoscope to locate the squeal. If it is the AC, you can get a belt that is shorter, and bypass the pump, since it doesn't work anyway. Try some belt dressing first. When the engine is running, just spray it on the inside of the belt.

Yes. Squeal is from a/c compressor. Belt too loose.

No, The fan clutch will not make a Noise it will just not catch the Fan to Cool. It is the Belt. Make sure that it is Tight. You may have a Bad Belt Tensioner if it is not.

Belts stretch from new. Try to tighten them again. Hopefully the are not glazed or this may stay with you.