Ferrolli Falcon 2 bolier - Screaming noises. ?

Hi, I have a screaming noise from my Ferrolli Falcon 2 bolier whenever the hot water is running. The noise is intermittant but is there 80% of the time when running the hot water tap? This does not seem to effect the operation in any way and I am still getting Hot water.
Please could anyone advise me what this noise may be cause by?
Thanks