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You are correct. Only facelifted E90 330d's (2008-onwards) were fitted with the N57. I must've mis-read the original post and missed that it was a 2006 model which means that it does in fact have an M57.
In which case, I agree with you: the valve cover gasket is the leading suspect.
Apologies to OP for any confusion I may have caused by my poor reading skills.
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