Volvo addressed that with Pentosin CHF 11S synthetic power steering fluid. There were weights put on the cooler loop forward of the LF wheel that also dampened the noise and the hydraulic surge that makes the cooler loop an lines rattle. The fluid was the most successful thing they did.

I have done something similar to this on my 1988 Volvo 760 with B230FT. I installed a breather filter on top of the valve cover as you suggested. I then installed a Toyota Supra Twin Turbo PCV valve (13 mm inlet x 10 mm outlet) into the hose that comes from the block mounted oil separator and connec...