My 1997 Volvo 850 T-5 makes this noise when I'm driving and come to a complete stop and hold down the break. If I put it in neutral the noise stops and when I put it back into drive; the noise starts again. I just recently changed the pcv system so I can't be that, but I can't figure what it is.

https://youtu.be/pkjEbZGQMQU

 

could it be vacuum pump related??? use a tube as a mechanic's stethoscope to listen around the right hand side of the engine and then listen around the accessories (power steering pump, alternator, AC).