Here we go
Just popped out to lunch and about 2 minutes from the office I suddenly noticed the engine sounding way louder and "harsher" than it normally does. Not all the time, only when giving it slight throttle. If I prodded the accelerator a bit whilst above 2k rpm then rather than just taking off immediately like it normally does, there was a slight hesitation and then it went but I also noticed that, once it had "gone" and was properly under some load, the engine sounded normal again.
Stopped at the shops for 5 mins and when I came back it was still doing it so I decided to take a trip to the dealer as they're only 10 mins away. Stopped the car, popped in and came back out a couple of mins later, started it up and revved and blipped it quite a bit whilst they peered under the bonnet but it sounded normal and they could find nothing wrong. Drove it back to the office and it's been absolutely fine ever since.
Baffling! Will obvously keep an eye on it to see if it happens again but what's really strange is how it started so suddenly and then stopped just as suddenly. Almost suggests a "software" rather than "hardware" problem, such as the computer had a brain fart for a while. Surely a mechanical problem wouldn't just vanish like this unless something was loose under the bonnet, which the dealer verified wasn't the case.
Don't suppose anyone else has had any such issue with the 170 diesel engine?