when I turn on hot water tap, I hear a big noise, like a explosion?

My boiler is (worcester bosch 28i RSF), and it is 6 years old by now, regularly has been serviced and works with natural gas. For the last two month when ever I use hot water after the ignition time I hear a unusual noise which is like a small explosion and it shakes the body of the boiler which is worries me. Could you please advise me how I can fix this problem. These are some more details.

CAT I 2H GC NUMBER: N.G.4731154 BENCHMARK

COMBINATION BOILER OUTPUT

TO DOMESTIC HOT WATER TO CENTRAL HEATING

MODULATED CONTROL MODULATED CONTROL
MINIMUM 8.5 KW MINIMUM 10.5 KW
MAXIMUM 27.5KW MAXIMUM 27.5 KW

Thank you very much for your help
Best regards

MANSOUR GORJIAN
May 2010