What is the normal sound a combi boiler should make?

I've recently had a new Worcester combi boiler installed. It is fitted in the old airing cupboard upstairs in the house. Central heating side is working fine and there is hot water, but it does make a strange sound when it's on......like a louder version of a cat purring. But since i'm not sure what sound is 'normal' for one of these boilers, it might be okay. The fact that it is wall mounted in a cupboard may make it sound worse than what it really is.