mentions, using a stethoscope (versions are made for tracing engine noises) - while the engine is running to trace down the location of the whistle / sound will narrow down what to look for.
The other item I forgot to mention in the prior post is whether the PCV system has a leak. This wouldn't have been an item involved with the oil change (the shaker base plate covers this up, normally base plate isn't removed for oil change service...).
another item to check is the MAP sensor (rear of intake) if that O-ring seal has a leak
From what you describe, its an item that has a constant high vacuum source at idle, and that points to a vacuum leak somewhere.
Today's cars don't have the myriad vacuum lines we used to see on the 70s/80s/90s engines, so there's fewer items to trace down.
You used to have vacuum lines for:
air diverter valves [A.I.R. pumps]
distributor timing vacuum advance
exhaust heat valve / crossovers
thermostatic air cleaner heat control
coolant over temperature valves
MAP / BARO sensors
turbocharger waste gates
and so forth. There was a lot of "spaghetti" under the hood and the fittings or hoses would deteriorate and leak. The computer controls eliminated a lot of vacuum / mechanical system that used to be in many applications.