Oh my son of rajab.
I can't believe some gutless jealous person has said this about your R33.
By the sounds of the letter, I don't think this could be the new neighbours, considering they refer to the 2-3am business..
Do you have any enemies? Someone that doesn't like you?
I think this letter is just trying to scare you, as they are not willing to show who they are, or let alone refer the letter to you personally.
My guess is that it's just some old person who has nothing better to do but be envious of your car. There is one like this down the road from me.
Your best bet is to honestly do what has been mentioned, basically take the letter to residents who you might think would have something against you, and ask them if they have a problem, and work something out.
Honestly though, who does this? Someone with plenty of time on their hands and is annoyed of a noise they might hear 5 times a week? For around 10 seconds.. I smell old person.
Either that or someone with a V6 Commodore or Falcon who thinks their the crap and hates your car cause it's better and out-performs their common everyday family mover.
Best of luck mate, and it's really sad to see stuff like that...
A SAU Cruise won't do much good in my opinion, considering it will only cause crap with other residents, and if someone does mention something to fellow residents, they would go "ahh yeah I remember when 20 of his mates came over with their loud exhausts and doof doof music and pissed us off"..