Ok, we drive dervs, the engine is never going to sound anything other than like a hushed tractor, especially when cold.
I have noticed a slightly different noise over and above the usual drone of the engine. It's a much higher pitched, high frequency rattle. If I had to liken it to something, I'd say it sounded like tappets.
Only seems present when warm, but may just be because the rest of the engine noise has settled.
Does anyone else have this?