300tdi strange air leaking noise

Hi guys,
Hopefully you can help me. My Defender 300tdi has developed a strange noise. I have looked on other posts to try and find if anyone has had a similar noise but can't find anything.
The engine sounds sweet as a nut at idle and up to around 25mph. After this speed regardless of gear or load on the engine there is a very load "airy" wooshing sound. As if air is leaking fairly substantially from somewhere.
It sounds like it's coming from behind the middle of the dash and resembles the heater fan turned on and blowing very hard.
The noise gets loader the faster you go and is unrelated to engine revs or gear, and presumably turbo also. The noise doesn't go away even if you coast out of gear down a hill.
Has anyone any ideas? I maybe thought air intake hoses and intercooler hoses etc so checked them all and they are fine. That also wouldn't make sense to me as the noise would fluctuate depending on revs and air being sucked into the engine.
I also read somewhere that a guy had a leak in his brake vacuum hose. However I have checked these and they seem fine too. No problems with brakes or anything either.
Hope you can help shed some light on this. The noise is getting a bit rediculous, I know they aren't quite vehicles by any means but this noise isn't right at all.

Thanks

split or loose turbo/ intercooler hose is were id look first

Thanks - I've removed them all and didn't find any splits. Also all the joints seem solid too.

air filter ok and std

I have a k and n so I thought this might be the culprit, but I swapped it out for a mates new paper standard filter to test. The noise was still there.

Any other thoughts folks? It's not a normal noise even for a defender...

If this only happens when moving then it must be wind noise on something catching the wind that shouldn't.
Do you have engine covers underneath? Has one moved a bit and is flapping about? Loose wheel arch inner or eyelid. Something like that.

Seal on the vent flap?

I'll look into if it might be whistles caused by things like the above. It's a good shout.
It doesn't really sound like a whistling noise it sounds more like lots of air escaping from something under constant pressure. It doesn't fluctuate with high speed gusts of wind from different angles - but I'll have a look and report back.

I get similar, but it's easy to see .. well so easy you can see the road through parts of the floor ..

Seriously, there are a few large grommets in the bulkhead, it could easily just be force of air through one or more of these.

If it's happening when coasting (engine at idle) then it's not likely mechanical. Take the carpets/mats out and take it for a spin to help pin it down. Also try moving the front vents when it's happening - and as above , the bulkhead is full of holes, which should be plugged.

A little update.... Alternator has packed up so wasn't able to do any fault finding this evening.

However looking more closely at non mechanical wind gaps, there are unite a few possibilities... I have a roll cage, raised air intake, led lights bars etc so when she's running again I'll plug all the possibilies up individually and see if the noise decides to stop annoying me! Haha

Finally fixed the alternator and did some more looking into this noise.

Looked at around the bulkhead and floors etc for holes they are all plugged up really well. I think the last owner must have done a bit a wading (previous owner was a marshal at Billing).

I also stuffed wrags into the gaps between the rollcage and the body / light bar etc and the noise was still there. I also put wrags between the rollcage and the front vent flaps noise was also still there. I tested the noise by opening and closing the front flaps too which unfortunately didn't make a difference.

Interestingly though I opened the bonnet when I got home (engine switched off) and there was a slight air leaking / sucking noise coming from the brake servo - perhaps there is a slight leak on that? Would that make that much noise though?

Try getting up to speed and pressing the brakes and listen carefully to see if it goes as you're using the pressure, or try running without.

Thanks Fraser - I've tried that and it doesn't make any difference to the noise. It's getting incredibly frustrating now - it's like it's getting loader. I'm sure it's not but it feels that way. I can't drive anywhere for any length of time without it seriously annoying me.

I've uploaded a video to youtube that I took just now. The sound isn't great but hopefully you can hear what I mean. For the clip I started driving down a hill (1st, 2nd, 3rd gear) and then dipped the landy out of gear and coasted down the hill at around 30-45 mph. This reduces the engine noise and should make the hissing / blowing / breathing noise more prominent. Link below;

Difficult to hear it as a hissing sound through the video, mate. Sounds more like a tinny rattle, if I'm tuning in on the correct bit of noise.

viscous fan !!!!

Do they make a tinny rattle when they go?

normally not .it does sound like a constant fix fan noise ,,if im listening to the right noise ,,,it is after all a ,,land rover ,,,!!!!!!!!!!!!

Didn't think so. It's tricky being detached from the vehicle and listening to the sound track. More annoying for Forbes though!