I know it's really hard to play "diagnose the noise over the Internet" - and when I say hard, I mean downright impossible, but..

After getting out of the car (engine off) recently, I've noticed what I can only describe as a tiny, tiny foghorn noise, coming from under the bonnet - as if the Borrowers were living under there and needed to warn passing ships of fog.. popping the bonnet it seems to be coming from the front nearside corner and I think it might be a rather noisy small fan.

Since it's been cold recently, and this was short journeys, I'm thinking perhaps the diesel fired heater? Are they generally 'audible' or should it be whisper quiet?

I'll try and get a video next time..  2014 BMW 530d Touring, 2006 BMW 650i, 2018 Mini Cooper S

Could be the fuel burning heating coming on or going off,I cannot remember what mine sounded like on the TDV8. Andy.
21 Ford Ranger
Gone but not forgotten
Focus RS Red Edition
2015 RRS AB Fuji.
2014 RRS SC Santorini
2016 VW Amarok
2014 RRS AB Fuji
2012 RRS Red Edition Fuji
2015 Nissan Navara
2012 RRS SE Santorini
2008 RRS White
2007 RRS Santorino

Yeah - I'm fairly sure it's that .. but you know what it's like; "New noise? What's broken now.."  2014 BMW 530d Touring, 2006 BMW 650i, 2018 Mini Cooper S

Presumably the FBH would only still be running if you had just made a short journey. Whenever I turn off my TDV8 there is always a high pitched whine which lasts (I think) about 5 minutes. I am told that this is just the systems all settling down after the journey. Is that the sort of thing you are talking about? Robin
2008 Range Rover Sport TDV8
Gone:
Jaguar S-Type V8 4.2 Sport
MGZTT 190 2.5

Nope - I know the noise you mean; since it's always done it, I've always assumed that particular noise was normal* .. good to know it is!

This one is a much lower tone and really does sound like a lilputian fog horn - which is what made me think of the FBH as, presumably, that works like a gas boiler and has a fan to force combustion air in? Next time I pop to the shops in it I'll take a video

*As in I was assuming it was normal because otherwise it would be expensive. AKA I was Ostriching  2014 BMW 530d Touring, 2006 BMW 650i, 2018 Mini Cooper S

 Robin
2008 Range Rover Sport TDV8
Gone:
Jaguar S-Type V8 4.2 Sport
MGZTT 190 2.5