I would bet on a leaking intake manifold gasket or throttle body base gasket. Basically, extra unmetered air is getting in through one of those gaskets. Get a spray bottle of water and spray it around those areas liberally while the engine is idling. If the sound of the engine changes, or the engine dies or nearly shuts off, then you have found where the leak is.
Let us know what you find.
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Thursday, November 9th, 2017 AT 7:34 PM