Spooky, high-pitch noise on 1994 940 wagon

[RedBrickRoad]

When I turn on my 1994 940 wagon, And put it into gear, occasionally, I hear a high-pitched "whirring" noise, kind of like a theremin (i.e. kinda spooky sounding). It is not very loud and it changes pitch as the speed of the car increases. At some point, perhaps when going 15mph (25kmph) or faster, it stops all together, or at least I can no longer hear it.

Additionally, the noise briefly changes pitch (a bit louder, or higher??) when I put the car back in park.

I can only hear the noise when I'm inside the cabin.

I'm happy to provide any additional information, I just don't know where to start. I've taken one entry-level mechanics course at an adult extension school and this is my first car, so be gentle with the vocabulary

[jimmy57]

To be better able to answer tell us more.
When engine is idling out of gear and then in gear does the noise change if you turn steering wheel either direction? (might indicate power steering pump noise)
When out of gear and engine running at idle does turning on headlights make noise louder or quieter? (alternator or alternator belt noise)
Does noise go way if engine has run for more than 15 minutes of driving?

[RedBrickRoad]

Great questions.

1. when going from neutral/park into drive there is a momentary change in sound. Nothing changes when the wheels are moved side-to-side.
2. No change in sound when the headlights are switched on.
3. Only driven it once for more than 15 minutes, but it was before noticing the noise (see below for why not more driving yet).

Further things I figured out today:

1. AC compressor is not working. When we connected the battery, the compressor started spinning, so possibly a leak somewhere in the system, or could simply be out of fluid. Going to check that tomorrow.

2. The pulley connected to the crankshaft wobbles some, so it's possible the wobble translates, inside the passenger cabin, into the noise. Previous owner had crank seal replaced, so it might be related to that.

3. Found a small leak in the power steering pressure hose, so going to replace that tomorrow and see if that affects anything.

[Jvell960]

Perform the pcv glove test on your engine, engine off, remove the oil fill cap, place a latex glove over the inlet use a rubber band if needed, start engine look for the glove to blow up or collapse, if it blows up then your problem is the pcv system is clogged and your having pressure buildup in the crankcase. OR remove the oil dipstick, if smoke blows out same problem. This will cause a high pitched squeal as the air is being blown out pass the oil fill cap. Service the pcv system ASAP as this can cause your engine seals to blow out! My car 95 960 was doing the same thing and I've never heard of a pcv system (on Volvos) consists of hoses and a box attached to the engine block FCP sells the kit. If your car doesn't have this lucky you, just replace the pcv valve. FCP also has vid on replacing the system components Good Luck Bro!