Need help please!

Hi all

Ive noticed the engine is making a strange noise. when I rev it up and the revs are coming back down, it sounds like a knock at low revs before it hits idle. Also if the engine is running and I take the oil cap off there seems to be a good bit of steam coming out. Plus I've been told the engine doesn't idle nicely? It's seems fine to me. I assumed it's how the boxer Diesel idles lol

Any more info needed just ask and any info received is greatly appreciated as I'm hoping it's just boxer noise and don't want to get rid of it!

Thanks

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