2007 Mazda 3i 2.0L P2187 Whistling
 

First there has been a whistling/light chirp sound that starts and fades quickly when releasing the accelerator.
Then a persistent whistle developed at idle and I now get a CEL P2187 System Too Lean and Idle Bank 1.
Neither sound can be heard while accelerating and I had suspected a vacuum leak. The car seems to idle and drive fine from my perspective.

The persistent sound appears to be coming from under the throttle body. I had already cleaned and replaced the gasket on the TB so that was eliminated.

Personal mechanic suspected Intake Manifold. He removed, cleaned, tested actuator, replaced PCV valve, PCV hose and IM gaskets but the sounds and CEL continue.

Mechanic believes there are some brass fittings/bushings that also tend to wear and may be causing the issue in the IM. Can't find any vacuum leaks and hate to replace the IM as a guess...may cost 1/2 its value.

Thoughts?

OK, finally found that pressing on the actuator changes the noise. Going to assume the plunger/rod/bushings for the flappers are probably shot and I need a new intake manifold.

Anyone know the part number for a Dorman or 3rd party IM? I can't seem to find it anywhere for the 2.0L engine. The OEM version is at least $500 if not $750 for the replacement.