This is the RWD forum Try the AWD forum. There is a pulldown menu at the top. Click on AWD then V70. You'll get more responses there.

Personally I've never heard of a popping sound coming from a clogged breather box/oil separator. You can pay your mechanic to service the breather box but that is a rather expensive guess. Does it run fine otherwise? Good power and smoothness? Any check engine lights? If everything else is okay it may simply be a bad vacuum line. If you find a bad vacuum line you could ask your mechanic to service the vacuum tree which is near the breather box. My 850 (same engine) with 190k has never had that problem. Don't worry, the 5 cylinder engines are great and will go 300 or 400k if taken care of...