Hey, this is my first post here.
A little while back my Volvo started whistling and I see on the forums that it was most likely the PCV/Oil Trap. I bought a kit that included hoses that branch out from the PCV. After installation it was quiet for a couple hours, then the squeal came back.
Any suggestions on what to check? The dipstick or engine oil cap being removed still make the sound stop like before.

 

try doing a rubber glove test (latex glove over the oil filler). If it inflates you still have positive crankcase pressure. If you replaced the oil separator already, then its likely you have a clog in one of the hoses to the block or in the ports within the block and need to replace the lower hoses and rod out the block (common issue with the older I5 cars like the 850s)

 

Just pulled out the PCV box, all hoses are clear. Reassembled the box and it still whistles. Tried the rubber glove test and it's pulling the glove.
What exactly did you mean by rodding out the block?

 

Positive pressure (glove inflates) means your PCV system is clogged. My reference to "rodding out the block" is what can happen on older cars where the ports in the block are clogged by sludge and they need to be physically cleaned out (ie with a screw driver etc or similar rod).

Since your car is pulling the glove in, it means your PCV is open, but it may be pulling in too much vaccuum. - ie possibly due to a bad diaphram in the oil separator or it was installed incorrectly. shop techs will use a vacuum gauge to determine if the car is in the correct range.

I'm not sure if your model has any form of a check valve (or if that's internal to the oil separator) but I'd look into whether the oil separator is the correct part # etc.