First things first.
Find out his full name, collar number, station he works at and also his superiors details, if you wish to make a formal complaint or take this further.
Next thing is there is always two sides to a story; normally officers won’t just barge over in plain clothes, flash their warrant card around and give you a dressing down in the street for having your engine ticking over.
A possibility is that he could still be in his probation period before coming an officer and is just throwing his weight about due to his new powers?
However, regardless of the situation is unlikely that he would come over and start to shout at you for having your engine on ticking over, there has to be more to it than that?
How often do you have your engine on for? Honestly do you rev it on the drive way? Do you have other car owners at the property with their engines on?
In the past I have warmed up my car on the drive way and popped inside to get some bits, it is surprising how far sound travels from the GTiR. For other residents that are not “petrol heads” it can become very tiresome and annoying. If there are complaints from other residents about your car and its noise, then something can be done about it, if it is taken up with the police…
I also find it very unlikely that he will follow up his comments about having the traffic dept come round to your house and perform a db level check.
MOT testing says that your exhaust has to be 98db’s or under. From the factory Mongoose units are 98db’s, but over time as I found out they can get louder (Oulton Park = 110db’s). Just out of interest, where do you keep your car at night?
From what you’ve said it sounds all a bit too excessive and heavy handed; unless you’ve been a tool, revving your engine or booting it off down the street?
If I were you, keep a cool head; don’t do anything to antagonise the situation. If he comes over again, keep your cool and ask him lots of questions about why he’s talking to you, what is his out come, his reasons for targeting you and don’t forget to get all his details?
The more info you can get out of him the better, you’ll know his motives, his attitude and if you need to go further you have an understanding about the situation.
Don’t get cocky with him and if you can negotiate an out come then even better as it could put you in control and you could dominate the conversation.