I have a video so you can hear for yourself:
The noise doesn't happen at idle. It is most noticeable just above idle, but it's still there at higher revs too. This was in neutral (it's a standard), but it happens under normal acceleration as well. This noise started happening yesterday, and I had never heard it before then. Upon my initial research I thought it might be the that common crank case breather problem, but that doesn't seem to be it. The car does have 193,000 miles or so. I've owned it for the last 10k, and this is the first real problem it's had beyond normal maintenance and me trying to take extra care of it.

You guys are certainly more knowledgeable than I am, so I was hoping somebody would know what it is! It seems to be happening very near to or inside the engine itself. I took off the timing belt cover to get a closer listen but didn't really come up with anything else.

 

when was the last time you did the timing belt/water pump/accessory belts? You may want to try a few things - have somebody turn the AC on off while listening (to add/remove a load from the accessory belt), see if the sound changes when turning the wheels, turn on/off lights/blower/rear defrost (load/unload the alternator). see if you can find a mechanics stethescope and listen around various points - alternator, power steering pump, vacuum pump (if your model has one?). Next would be to remove the upper timing belt cover to see if that makes the noise louder. Finally you can even remove the accessory belt to see if that changes anything. My sense is its probably an idler wheel/tensioner or a drive belt accessory (AC/PS pump/alternator). If you have gone more than 7 years or 100K miles on the timing belt, best to change asap since a failed idler or water pump can snap the belt and slam all the exhaust valves...(ie major rebuild)

 

There are 2 components that run using the serpentine belts. The alternator and the AC compressor. My money is on alternator use a stethoscope to listen and see what part is the loudest

 

That sounds like the "oil trap / oil filter" assembly, which is part of the PVC system. They get clogged over time and are replaced as a unit. Not hard to do, but you need to get it verified by a mechanic. Change it now or you risk blowing out seals on the engine, which will be a MUCH bigger problem.

 

My 02 S60 has a similar noise. I replaced the power steering pump and the serpentine belt tensioner, but the whine is still there. I was wondering if you were able to find the source of the noise?

 

Did you manage to find out what was causing the sound?... as my S40 made the exact same sound yesterday, but when i took it straight to the mechanics the sound disappeared, yet came back slightly this morning on start up only to disappear again after driving for about 10 mins

 

There are several things that can cause a very similar noise.

The right answer to the dilemma is to spend a few (literally) bucks at Harbor Freight and buy a mechanic's stethoscope. Then, it's a trivial matter to poke around on the suspects (power steering pump, alternator, tensioner pulley, A/C compressor) and find the source of the noise.

 

it had a new compressor about a year ago, steering pump seems fine...as for the water pump or cambelt, pully etc not sure

 

Read my previous post. Proceed accordingly. ;-)

 

If it is the oil trap causing the noise , you can take out a little the oil dipstick, the sound must stop

 

I helped a guy with an Audi - his car had what sounded like a horrible mechanical rubbing / screeching noise. I actually poked around for a couple minutes with my mechanic's stethoscope wondering why I couldn't find such an obvious noise. Then the light bulb (the one over my head, not the CEL) came on, and I pulled the dipstick and the noise stopped immediately. It was just air being forced through some gasket somewhere in front of the engine, caused by a simple bad PVC diaphragm tear. I swear you could hear this car coming for two blocks - one of the stranger things I've seen.

 

That does not sound like a failed PCV which is easy to check as mentioned , my bet is the alternator they do give problems but could be any other bearing in many areas , as mentioned get a stethoscope

 

the issue was the Alternator, all fixed now