I get this too but not in my scoob. I get it in my peugeot 106 diesel. It sound like water being sprayed down a tube somewhere behind the dashboard. Its done it since i bought it so not bothered.

it will not cause a problem it's just annoying

they even put bleed nipples in the top heater hose on foresters which seems to work but i don't know why they don't do it on imprezas

I get it too on my 03 sti. Seems to happen when i put revs on just after start up.

As mentioned, top up the reservoir down by the rad and check the rad level and top up while cool, remove header tank cap and again top up, give all the pipes a squeeze. Then with the cap off start the engine, ensure heater is turned to full heat in car, allow the engine to run up to temp and rev it a bit to dislodge trapped air. You should see bubbles and the tank will over flow a bit, keep the engine running and allow the rad fans to kick in 2 - 3 times.
Once at this stage turn off the engine and allow to cool, top up as required once the coolant has cooled and keep an eye on the levels over the next few weeks.
Dont add cold water to the coolant when you are running up to temp, if you need to add water due to a big air lock use warm water.

fixed mine
 

05 sti - had the same thing a bit ago, was driving me mad. Nice chap on here said top up my water - so I did and it stopped. Make sure you squeeze all the pipes through and let it run for a while to check the level is correct.

Give it a go.

easy to get rid of,my 01 wrx had it...take coolant cap off leave it off overnight you will notice the next mornin the level will be low,top up to maximum fill level start car and leave to run for a while,replace cap,and happy days noise gone.

have done all these bar filling the heater hoses seperately and clamping them.....any other ideas how i can rid of this bloody noise!!???...its driving me crazy!

ps.....i havent done the hose clamping technique cause i dont quite understand what he meant....any ideas much appreciated!

You tried running without the header tank cap on and squeezing the pipes yet?

silly question, but what do you top it up with, can you just add water to top up or do you have to have coolant in there too. and which colour coolant, Last time I bought some, there were two types

You can use water if there is already coolant in there.

If you are going to buy a different coolant and you are unsure what is in there make sure you flush the system first.. Also you should buy coolant that is suitable for aluminum engines/rads..

yeah davey,have tried that but no joy.when u say the header tank do u mean the radiator cap?i have ran the engine with no radiator cap on until it overflowed,and squezed all the pipes.then let it cool off and filled the radiator and resevoir.still have the water running noise,and car is still using coolant-not a lot,but has used about 700ml im 2 weeks.u think this could be the airlock working its way out??.....how much u think i should put in before i need to take it to the mechanic?(how big would a big airlock be??....2l?..il?)

i have also left the rad cap off overnight and no luck.

you have to remove the heater hoses where they attach to the engine just under the intercooler, then holding them both verticle fill them both up with water this fills the matrix and gets rid of the air. then you need to clamp them off this is to stop the water running out when you reattach them. you will need 2 largish clamps as they are a good size pipe and you have to clamp them just before where the retaining clip sits that holds them to the engine. after you have done it a few times it second nature.
you then need to refill the engine with coolant as when you first removed the pipes you will have lost a fair bit. so fill the engine and last of all remove the clamps. it's a bit time consuming but it does work i normally do it when i drain the cooling system on a service.

i used to have this noise 2, it disappeared after going for some work at power eng, they had to drain system, so assuming as above it was airlocking, fully resolved after getting car back, i had not mentioned it to them when taking car there, now i keep my eye on water levels, but not had noise back at all, 3 months now

i still have that bloody noise!....cant get rid of it!...someone told me maybe water pump-what ya think?

It's the air conditioning isn't it? I get the same noise in both of my cars, and only when the A/C is switched on. I also used to get it in my old car which had A/C.