It could well be a vacuum leak from a broken or detached hose. As they have just done a oil change, they may have knocked a hose off. The likely one is the vacuum hose that runs from the right hand, back corner, of the inlet manifold, down to the fuel pressure regulator, when they checked the front diff oil level.
You can trace the sound, using a piece of hose as a stethoscope, to find where the sound is coming from.
Harvey.
