Alex, I have had the same sound off and on in my 300D. Try this, wht the engine idling, place your hand on the air cleaner, press down and see if the sound goes away. When I have a air cleaner mount broken the air cleaner rubs against the a braket next to the vaulve cover causing the chirp. (goes away at anything over idle speed)

Look at your other air cleaner mounts and see if one or more are not still in need of replacement. I usually replace all three at the same time. Hope this helps.