Ford Repair: 98 Mustang Squealing Noise - 6 Cylinder - Auto, crank shaft, o2 sensors

Question
Recently my mustang has begun to squeal on cold start - I have replaced serpentine belt, and water pump, tightened crank shaft pulley bolts and still no luck -- I did apply stp oil treatment and that made it quieter but not completely gone - I do know that some o2 sensors need replacing and am wondering if that could contribute to squealing noise; I have run the car without serpentine belt and noise has persisted - belt is not the issue from what I can tell; I'm at a loss - car is now at 125,000 miles - hope this is enough info to give you some idea of what's happening - Thanks


Answer
Hello and thanks for the question...I'll try to help.
 From what you have told me...the noise is still present even with the drive belt off--correct?
 Have you checked the oil level in the engine? It could be something like a rocker are that is starving for oil. Since you have said that you applied STP oil treatment, and the noise went away..... this would indicate that the noise is an internal engine problem. I would suggest you change your oil and fill it to the proper level.
 As far as I know O2 sensors won't cause a noise.
Hope this helps,
Erik