If you can't identify the neighbor, pretty much the only thing you can do is go to management. Six in the morning is pretty early for loud noises.

It could be that management can't help at all, or it could be that they can ask the guy to get his car fixed (if it is in need of repair), or maybe move his car to a distant corner of the parking lot. You won't know until you ask. At least you will have brought the issue to their attention.

And if they say they can't do anything, I'd encourage other neighbors to complain. Sometimes management won't address a nuisance issue like this if only one person complains. But if half the complex starts complaining, they'll realize they have a problem on their hands and at least try to do something.

Sounds like he may have a straight pipe on there. Most of those guys do that modification for sound/performance. Either way, it's annoying. Depending on what state you live in and the inspection laws, you could even go as far as call the police and report a noise complaint if the landlord doesn't end up doing anything. The guy could face a violation of the emissions laws in your state even if he has the sticker to prove he passed. Most cops know an illegal exhaust when they see/hear one.