Easy fix is to buy/make a silencer to stick in the end of the Exhaust when you ride in the yard.There are some on ebay that you can get,my mate copied one of them and milled it up at work and it definitely makes it quieter.It still can be heard make no bones about it but it is alot quieter and actually cleans up the note a bit as in the case of my mates bike which sounded like a wet fart before we fitted it up yesterday and we reckon actually improved the performance of his bike(at least on the tight track) a bit by giving it a bit of back pressure.
If you've got a bike which already has a decent baffle then it should quieten it down heaps,I'll let yous know once mine gets made up and see how it quietens down the xr,hopefully enough to make some jump sessions under lights a possibillity.The neighbours to our track are far enough away not to be bothered during the day and hopefully this might make it far enough at night too.
But it sucks that you can't even ride in the privacy of your own backyard without the cops rocking up,used to be able to make pretty much all the noise you wanted when I was a kid as long as it was before dark pretty much and no-one cared.Hell we used to ride up and down the street and no-one was bothered,not even the copper who lived at the end of our court.