1.5dci laguna 3 engine popping noise from oil cap

Hi all. I went to look at a 07 laguna 3 1.5dci with 167k miles today. it drove great but I took off the oil cap while it was running and I could hear a popping noise and it was blowing a bit. Is that normal for those engines or could it be a sign of a bit of engine wear?

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1.5dci laguna 3 engine popping noise from oil cap