alpha,ocean 240 eco.noise?

Hi, just wondered if anyone could help with a noise coming from my alpha ocean 240eco combi boiler. All works ok apart from when wifey runs a bath.when hot water tap is turned off,
(after about 10 mins)the boiler makes a short growl noise,then gas turns off.seems like boiler is over heating or could there be air in system?Can hear bubbling coming from top half of boiler for a short period afterwards.Has been doing this for about 2 months.hasn,t blown up yet.Wifey would be pleased if we could find answer to this problem as i would also...i prefer her when she,s clean.lol
Thanks for any help.
Dirty