The noise when the oil filler cap is off, certainly sounds odd, but so does our 2.7, so I wouldn't worry.
It's has been making that noise for 300k km.
The other problems would point to other issues though.
There are a couple of well know failure points on the cooling system, one is the coolant manifold in the VEE, the other is the coolant fitting behind the AC compressor.
Neither of these would put combustion gases in the coolant though, but they may have caused the problem.
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