This is still a commonly misdiagnose as a belt chirp especially on the 3.8l mustang but can sound like a general engine bay noise in the Explorer 4.0L OHV and Taurus 3.0L OHV especially. So before replacing belts and pulleys take a look to make sure this is not the actual source of all that noise people confuse the two all the time.

Quick Tips: #18 Ford 3.0L 3.8L 4.0L 12v Engines Chirping Noise - YouTube

 

Good video and explanation. Same thing on the 3.9 and 4.2 Essex engines. I was actually getting an occational idle stall and periodic harsh tranny buck before mine started chirping. This happened on my way down to AZ. As soon as I started hearing the chirp, I replaced mine when I got to our house in AZ. Problem solved.

 

I have only had that buck stall issue on the 3.8l too which is weird cause I have seen allot worse sounding ones on the 3.0L.

 

Mine would chirp 2-3 times every couple of minutes or so, but not very loud. When I pulled it, it was pretty stiff turning it by hand and would actually bind up and not turn by hand unless I turned it counter clockwise and then back clockwise. I'm at about 159,000 miles and the 4.2 still runs like new and fixing to roll down to Florida for a week.