Miata moo-ing when I turn it off. Really kills the vibe when I roll up some place.

It's the IAC valve but not due to frequency. Only happens when I turn off the car and it shuts down.

Have Skunk2 throttle body and NB IACV. Using a cheap aftermarket IACV. Recently switched from NA valve to NB.

Question: do others with same combo experience this? Should I get a genuine Mazda NB valve?

Video, unplugging the valve to make noise and also shutting down:
https://youtu.be/nAlRG-8W10A?si=qKXxbke4hSf1s9Wg

 

I think that is your EFR's CRV (Compressor Recirculation Valve), aka the blow off valve, sucking in air past the diaphragm.

Try removing the vacuum line to it, capping it or pinching it shut, and see if the noise goes away.

 

I thought that at first, but I removed the intake tube and the noise is definitely coming from the IACV.

 

Does it make any difference if you change your IACV to "run valve before start"?

 

No, it's quiet when the engine is stopped or the valve has power. If the engine is spinning and the valve doesn't have power that's when it makes the noise. Like when turning the car off, the engine spins down and it makes the noise.

 

So if you put it in gear with the brake on, release the clutch to stall the car, it doesn't make the noise?

 

Just did this and it's quiet. I'm guessing it has to do with the airflow or vacuum or something when the valve is not energized causing it to flutter. I think I need to get a genuine Mazda valve and replace the cheap chinese one I currently have.

 

I had the same noise with my OEM NB1 valve when I first switched to the Link G4X, and it didn't happen with MS3. It only did it when I turned the car off in my case, and it happened every time. I increased the frequency to 500Hz and the noise went away. Later when I did some wiring work in the car I found one of the pins to the IACV was loose and came off the wire almost by itself. I never thought much of it and haven't tried a lower frequency since, but your post made me wonder if one of your pins isn't properly crimped or is slightly loose since you swapped to a NB valve (assuming you already tried frequency changes as well).