It is officer discretion unless the exhaust is marked 'not for road use' or similar.
Be careful though as if the neighbour does report it then that is independent and you could end up with a s.59 Police Reform Act warning.
The warning applies to you and your car for 12 months so even if you change/borrow a car and get a complaint it still applies. Likewise if somebody borrows your car.
You only get one warning and if another complaint is received about your car or driving causing harassment, alarm or distress the Police can recover your car which will cost you. Think it's £120 for the call out round here then £12 per day.
Cheers
Adam