This may be something that has always been there, and I just haven't noticed it before.

When I accelerate, I can hear a hissing/blowing sound, like the heating system fan speeding up, although it's not that because it happens even with the fan turned completely off.

I recently changed the reversing light switch, which necessitated removing the air box (replaced the filter at the same time), but I'm pretty sure it all went back correctly.

I put a lamp inside the top air pipe, to check for splits, but couldn't see any light leaking out from any splits, nor can I find any from a visual examination.

Power seems unaffected.

Any suggestions?

Thanks.

Check the airbox seals, hose clamps and that the lid is tight.

It seems quite coincidental that the noise started after working on the airbox.

May be worth checking the hoses again though.

Also check the large black hose to the drivers side of the engine (intercooler hose) they do split and cause the noise you're describing, Mine went last year and sounded like a hoover revving.

As I said in my OP, I'm not certain the sound hasn't always been there, and I just didn't notice it.

The inter-cooler hose? Would that be the black one which passes across the front of the engine?

If it is that, wouldn't it affect performance?

It passes up the drivers side of the engine from the intercooler to the turbo.

You may not lose any power in early stages of failure but mau notice the noise getting louder when reaching boost pressure (1500 rpm onwards).

In later stages of failure the noise will be much louder and you may get the engine fault warning on the dash plus a red light and resultant DTC codes referring to boost pressure on a diagnostic.

OK, so I've had another look.

I tightened up a few hose clips (none of which seemed particularly loose) and found a gouge line, about an inch long, in the rubber feed pipe to the turbo.

It doesn't seem, from squeezing it, to go completely through the wall of the pipe, but I guess it could be leaking, and with all the negative pressure involved.

I'll have a proper look tomorrow.

This pipe takes a lot of pressure from the intercooler into the turbo so tends to flex while on boost.

A tiny gap can quickly become a split.

My hose went from making a hoover noise to sounding like a air hose type noise quite quickly.

The split was about 3cm long in the end and I had to make sure not to over rev the car until I got it swapped out.

From Ford the hose is about £60, part (Finis) number 1496238.

Ford only sell the hose with the metal pipe attached which is a bit of a gyp.

You will save money if you buy a silicone replacement part off ebay and swap it out yourself.

Just be sure to put all clips back in the same orientation or they can rub the underside of the coolant expansion tank.

http://www.ebay.co.uk/itm/FORD-FOCUS-1-8-TDCi-MK2-INTERCOOLER-TURBO-HOSE-SILICONE-C-MAX-1496238-BLACK-/300949025636?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item4611f5b364

http://www.ebay.co.uk/itm/BRAND-NEW-INTERCOOLER-HOSE-PIPE-FORD-FOCUS-C-MAX-CMAX-2003-2011-1-8-DIE-1496238-/321195886948?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item4ac8c44964

http://www.ebay.co.uk/itm/New-Genuine-Ford-Focus-1-8-Duratorq-Intercooler-Pipe-Hose-04-2005-07-2010-/320827806571?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item4ab2d3d36b

http://www.ebay.co.uk/itm/NEW-GENUINE-FORD-FOCUS-TURBO-INTERCOOLER-PIPE-HOSE-1-8-DURATORQ-DIESEL-ENGINES-/190744153126?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item2c693ccc26

If you just swap out the rubber part for a silicone one then it also saves you having to swap over the air charge temoperature sensor.

BTW you can temporarily patch the hose with duct tape wrapped around and clips over the split until you can replace it.

Found it.

It is indeed in the top hose, on the inside curve.

I've repaired it with a patch from a bicycle puncture repair kit, covered with half a roll of insulating tape, covered with half a dozen layers of duct tape, all courtesy of £land. :)

I'm hoping it will hold until I can pick up a new hose tomorrow.

Just going to refit it, and test drive it.

Just been for a test drive, along a local NSL single carriageway, through town, and back up the motorway, and I can report silence (al-right not silence, but at least normal sound) has returned. B)

Rung it out to 55 in 3rd, and all is well.

Who'd have thunk it, that a split like that would make so much noise.

There's a lot of pressure shooting through a thin slit so it does make a racket.

Glad I pointed you in the right direction and you can get it sorted now.

Thanks for that Stoney.

BTW. There isn't an air charge temperature sensor, in the rubber pipe that has split.

In fact, there isn't anything in the rubber pipe, apart from the split. :P

Even easier then.

Just going by Ford's microcat.

They sometimes add components to pictures from higher specs.

Job done and, I must say, it's not a particularly difficult job, requiring no tools other than a flat blade screwdriver.

Given the cost of the parts :o , if you can, then save yourself some cash, by doing it yourself.

A couple of things to note.

1. Andrew Page don't stock the item (nor is it on their order list), so if you want it immediately, it's a main agent job.

2. The top rubber pipe isn't available from the main agent, on its own. It comes with the steel intermediate pipe, and the price is (wait for it) over £50. :o

Fitting it requires the moving of (though not the draining of) the expansion tank. This is made easier if you insert a slender blade (eg. a thin and wide screwdriver) between the plastic slot and the metal tang.

When replacing the expansion tank, fit it to the rear tang (the one parallel with the firewall), before fitting it to the offside tang (the one parallel with the wing).

The whole job shouldn't take more than 15 minutes.

Fancy doing a guide on how to do the job?

A fair few owners have had this problem any your insights would help them.

I could do that, but it won't be for about a week, as I'm now back at work (hence the urgency of getting the job done), and doing 12 hour shifts.

Should I put it in a new thread?

Would you like photos?

Will you be making it a sticky?

Knock one up and stick it in the Focus guides section.

Photos are good if possible.

If you stick a link on the bottom of your posts then everyone will see it.

As promised Stoney, I've posted the guide.

http://www.fordownersclub.com/forums/topic/52021-guide-to-replacing-intercooler-pipe-on-a-mk2-focus-18-tdci/

BTW. How do I go about adding it to the bottom of my posts?

I presume it goes in my Signature (as opposed to me having to C&P it into every post ), but how do I do that?

That's right.

Add the link to your signature so it's always available.

Good effort mate.

But where do I create/edit my signature?

Go to your profle name top right corner of the screen. click my settings, then signatures then add one from there...