They aren't always constant. Most diesel engines I've dealt with are a lot louder when cold, quiet down considerably once up to operating temperature, and can get quieter still after being worked hard. The quality of the sound is also going to vary depending on rpm and load.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.