I had this happen on my '96 850 some years ago, it was the rubber bend on
the vacuum line from the end of the inlet manifold to the crankcase
ventilation system.  On the timing belt side of the motor under the inlet
manifold just below the thermostat housing.  A real bugger to get at, let
alone see.  The rubber bend goes "soggy" and gets a hole in it and the
engine vacuum makes it act as a "musical instrument" reed. It also was
really loud.  I also got a
fault code, forget now the number but something along the lines of "engine
running rich under mid throttle".

I replaced mine with a piece of appropriately sized rubber tube the first
time and the second time (it got soggy again) due to the difficulty in
getting my hands in, I re-routed the vacuum line (hard nylon) above the
manifold to the crankcase ventilation system on the other side.  I tied it
to the fuel rail.

I makes a scary noise but it isn't all that serious.

To confirm, try and poke the bend with something (like a stick) while the
noise is there.
It should change it.

I posted this a few days ago, but it never appeared....

barry