If its a standard exhaust and passed a recent MOT you have nothing to worry about but what you need to do is keep a record of his aggressive behaviour and attitude. He has no right in a shared dwelling to be ordering people about and threatening people. Keep a record of dates and times and a brief description of what he did or said. This all helps if he complains to what ever authorities, he sounds like an ar*ehole who likes to intimidate people. As long as you ride in a manner to minimise noise on arrival and leaving there is nothing he can do. I would suggest if he continues to intimidate your girlfriend I would go and see the landlord to make a confidential complaint against the moron. Good luck.