2002 Explorer churp while driving only
 

My wife was driving home yesterday and the Explorer started to make a churp sound. It only churps while driving. You can park the truck or stop at a red light, churp goes away. If you rev the RPMs no churp. Doesnt sound like it is in the engine, as I already replaced AC pump, alternator, tensioner, idler pully. I recently replaced the upper ball joins too, as well as all 4 bearings on the wheels. My question is can it be the brakes that are churping while driving. The brakes on the are about 1 1/2 years old, and are the cheapest they had at autozone. Thanks.

Yes, that is possible. The chirp seems to come from the wheels or drive shafts turning.

Check if the caliper slides are free. With the wheel off grab the caliper and push it in and pull it out. It should move with ease about 1/2 " or so. When in doubt clean and lube the slides and the mating surfaces. In one car I had to use sandpaper to get rid of the corrosion and rust.

Did you happen to notice the remaining pad material on the front pads when you did the ball joints?

I figured it out, one of the bolts on the upper control arm came loose and the washer was causing the chirp sound. Tightened the nut up and chirp went away.