Whilst Citroëns are generally soft and wallowy, I'd say that the seats in Volvos are far more supportive and comfortable.
The DS series are usually slated for being too firm in terms of ride.
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Volvo is a good suggestion. Never had one but would like to see how it does noise wise. I will buy second hand so the C6 is probably within budget if the road tax is reasonable.
I'm assuming your looking for reasonably recent? - my V70 is a phase 1 2.5 5 pot turbo diesel, from cold it sounds like loose metal in a washing machine but above 2k rpm it changed quite remarkably to a lovely 5 pot burble which is really quite nice for a diesel, quiet it most certainly isn't although I do believe the later D5 variations are quite refined.
Let's hope so. I had a 2005 D5, and IMO the engine was the worst part of it. Sure, it was reliable, but instead of the claimed "power of a six, economy of a four", it rattled like a London taxi, and very much had the power of a 4 and the economy of a 6. It was one of the key reasons the car was sold.