If there is a banging noise from the HE I wouldn't class that as anything, if for any reason you cannot do the basic checks after working on it i.e cant analyse, gas rate, test fsd etc, then I would potentially at risk it as you cannot confirm its safe, but if you can and it sounds like a plumbing issue, I would just note it down and advise not to use it, essentially you're classing that appliance as immediately dangerous without knowing the reason?