What engine is this? Are we talking a BEW? That is a noise from the INTAKE. To confirm, turn engine OFF. Remove the intake boost pipe from the intake manifold. Remove lower wheel well splash guard. Turn the crankshaft clockwise with a breaker bar connected to 2-ft long extensions and 19mm 12pt bolt socket. Standing from the passenger side of the vehicle, use the breaker bar to rotate engine two revolutions. Listen for a sucking noise on intake. 'Funk!'.
This sucking noise it caused by wear on the cam. When the cylinder has both valves closed between exhaust and intake strokes, the piston drops creating a suction before the intake valve opens.
If that is the case, your cam is worn. Remove the timing belt cover, and if it is a BEW, the three screws that hold the EGR in place and the 20 bolts for the valve cover. Remove Valve cover and inspect the cam for severe wear.
We have been selling replacement cam kits for this issue that cure the problem for over `10 years. Sorry this is not the cheap option you may hope for, but the indications are plain on the cam. You will find galling on the base circles of the exhaust lobes. Probably in a severe case, the top of the lobes are apparently worn, and you can see the cam followers are dished. When the cam is removed, sometimes the cam followers have holes worn through the tops.
We have repair options to make it never happen again or others to just keep it going.