First thing to do is to give ES6T a large Thank You (somewhat more than delayed) for the help he gave me in figuring out my little problem on the road.

2008 V70 3.2 LT

Late September into early October we were on vacation through Ontario and then south to Kentucky. We stopped in north western of Ohio to visit and as we were coming to the hotel there was a horrific high pitched noise. I of course looked around traffic to find out what was making the noise. It eventually dawned on me that it was me. The noise was from the engine compartment of my car.
At this point a certain amount of panic set in as the nearest Volvo dealer was 1-2 hours away if parts were needed. Never mind the expense of potentially paying some one else to repair the car or get it to decent shop if I was not able to repair on my own.
The noise was loud enough that a gentleman in the hotel parking lot heard it from across the parking lot and came over to offer help.
Potential causes began to filter through what is left of my brain. Worst case failure problems of course popped into my mind first of course. Bearing failure of the components at the rear/accessory end of the engine was at the top of the list. Alternator, power steering, AC compressor, belt pulleys.

Started with simple diagnosis. The noise was only there with the engine running of course. Revving the engine changed the noise. It was loud enough that it was not possible to really find out the exact location of the source.
The car now has enough kilometers (140,000 ish KM) on it that a road trip toolbox and my VIDA and DICE travel on long road trips. Screw driver became a stethoscope. Bearings did not seem to be an issue. Hard to reach some of the components but I was confident the noise was not bearings. No noticeable excessive slack in the belt, not that the noise sounded like a loose belt. Oil level was fine so did not think that any thing would be wrong internal in the engine.
Over the course of diagnosing noticed that the noise would endure for a few seconds after the engine was turned off. This began to bug me enough that it became the focus. With the engine running it was nearly impossible to remove the oil fill cap. With it off the noise stopped. Put my hand over the cap hole and felt lots of vacuum from the crank case. Not normal.

Some thing was letting excessive engine vacuum into the crankcase. Revving the engine changed the vacuum level and noise. Noise continuing after engine shut down was the vacuum bleeding down.
Reason for the noise now known. Cause not known.

That night I sent ES6T a private message with symptoms and he responded very quickly which was very much appreciated. The answer was Oil Trap. Apparently there is a diaphragm in it that ruptures and provides the entertaining noise that I got.

Here is a picture of the new one on the engine.

20161012_164705 by ssicarman, on Flickr

Another closer shot. Take note of the round piece on the right of it.

20161012_164637 by ssicarman, on Flickr

Due to travel/hotel commitments and Volvo dealerships in the Louisville/Lexington area a quick noise reduction method of drilling holes in the oil fill cap the trip continued. I drilled the holes I think more to prevent potential damage to engine seals.
After getting as new oil trap and filler cap from Volvo of Lexington, cheaper than Volvo of Louisville and they had the cap in stock. Crossed the road and performed parking lot surgery. Took about half an hour start to finish and the noise was gone.

Original oil trap.

20161012_074423 by ssicarman, on Flickr

Diaphragm disassembled from the oil trap

20161012_074714 by ssicarman, on Flickr

The cause of the whole problem

20161012_074732 by ssicarman, on Flickr

Under side of the oil trap

20161012_074802 by ssicarman, on Flickr

 

what caused that?

 

I think that the diaphragm just reached the end of it life.