I cleaned the throttle body today, together with changing the air filter (K&N) and spraying the MAF sensor with some MAF cleaner. After mount everything back (I took the throttle body out), I let the truck sit for about 2 hours. Then I took it for a test drive. No engine light, however, I noticed a ringing noise, like the ringing of a bell.
Maybe the noise was there before but I didn't pay attention to it. Does it sound familiar to anyone? What could cause the noise?

 

I had a similar noise that had been coming from my a/c compressor clutch. As it turned out, the clutch and the compressor went bad. If you have a stethoscope, you can try to isolate where the noise might be coming from.

 

It seems to be the AC compressor clutch.
With the AC off, the noise is not there (today there is some wind, so it's harder to listen). If you got a minute, please give it a listen and let me know it's my illusion or not. Thanks a lot!

Both fan and AC off:

Turn the fan on, AC still off:

Both fan and AC on:

Keep fan on, turn AC off: