I have a 2006 Jetta with 90,000 miles. There is a low level noise which my mechanic seems to think it is in the engine or transmission. From the drivers seat it seems to be on the passenger side of the car. The noise is there when the car is stopped or moving. It seems more pronounced when you you give the engine gas. I hear it at 1000 RPMs and above, although the mechanic can here it on the rack. The sound is a pulsating un un un. If you have ever heard a fountain pump when the water is to low to pump, it is a similar sound. The engine and the transmission seem to work normal. could this be a bad oil filter. The filter is not a VW.
Use a stethoscope or rubber hose and listen around the accessories (water pump etc.).
It’s also possible to remove the serpentine belt and run the engine a few seconds to see if it goes away.
Does the frequency of the pulsating increase as engine rpm rises?
@insightful 's question is an important clue to the solution. And @circuitsmith 's idea about identifying the location with a hose is the first place to start w/a diagnosis. Of course it is not possible to venture much of a guess without hearing it, but my first suspicion is the accessories or their associated belt(s) and idler pulleys , esp the water pump. I’ve had failing water pumps make that kind of noise. This car probably doesn’t use a timing belt, but if it does then it shouldn’t be restarted until the timing belt is checked, due to the cost to fix the damage if it breaks when the engine is running.