You missed and important detail on the caller with the Nissan Pick Up that made a periodic and random chirp while driving and that detail was that it started about two months ago. If he does nothing, it will go away in about two months from now, but re-appear next May or June.
Its the Air Conditioning. For some reason, the compressor randomly freezes for just a fraction of a second and make a chirp sound. It drove me nuts for the first couple of years that I had my truck. It made the sound from the second year after I bought it new.
I have found that if you get the belt tight enough, the sound will stop. But the belt has to be pretty tight, tighter than I usually like to make a belt.
For some reason the chirp sounds like its coming from under the dash or just forward of the firewall. Occasionally it sounds like its coming from the driver side front wheel and sometimes you’d swear it’s in the door. But its the AC.
What was your diagnostic technique?
After the second year, I noticed that it only happened when the AC was on. Then I tightened the belt a little and it went away for the rest of the summer, but it returned every year and the belt had to be tightened a little more to make it go away. The original belt lasted ten years and I only replaced it because it was ten years old, I replaced all the belts then.