Hi please can someone tell me if it is normal for a new Baxi Solo HE condensing boiler to make sounds in operation that sound like pipes contracting - sort of expansion noises - they aren't regular, sometimes more than others and are not really very loud , Have had many problems so far with this boiler and I don't remember if this has been there since the installation. It just sounds like a small ball inside the boiler knocking every so often against the wall(hard to explain!) but I just want to know if this is normal or not. Thanks.