An energetic high pitched whistle coming from AC unit. It makes the noise intermittently as you run the AC. The whistling continues for a while after the car is turned off. It sounds like something that I would expect to come from a pressure or vacuum line. I have had the AC checked twice, and the mechanics can't find a problem with the AC system. They do hear the noise, but they say that they can't find a leak or anything else wrong with the car. Please help.
SOURCE: mazda mpv 2005 vacum leak
99% of the time it's a pcv hose, put your hand behind the intake manifold while the car is running and you should be able to feel it. it will be sucked in at the elbow on the manifold, this is not an easy hose to replace.
SOURCE: 1999 mazda 626 4cyl runs rough at idle , engine
You may have to dig deeper for a vacuum leak. It's possibly the intake manifold gasket. Try spraying some starting fluid around the intake manifold. Also try disconnecting the brake booster and plugging that hose, the diaphram in the booster could be ruptured, causing a vacuum leak. Check the PCV valve and hose too.
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