I'm kinda in the same boat as you as far as HD is concerned. I rode dirt bikes for years, took a 20 year break, and decided I had to ride again. Decided to get the CVO Street Glide (hey, you go big, or not at all, right?) and have been wondering what are "normal noises" ever since.
I was told one of those noises was piston slap, often the result of tapered cylinders from poor manufacturing. It sounded to me almost like wooden blocks being smacked together - not really a metallic noise. It started on my stock bike at about 2000 rpm and faded out at about 2200 rpm. Then, it came back from 2900 rpm to 3100 rpm. It seemed to be getting worse, and it was embarrassing riding past other bikers. I'm not sure if that description matches yours.
So, I went to two different stealerships, and they both said the same thing: "That's a problem, but the 110s are noisy. When it breaks down, we'll fix it under warranty." What a load of shyte. I didn't want to wait to break down in the middle of nowhere some day, so I just replaced the cylinders myself. That noise is gone, but now I hear more valve train noises.
As for the backfiring, mine did it, too. Plus stalling when hot, decel popping, noticeable throttle lag, and ran really hot. This is due to the lean AF ratio from the factory that allows the emissions to meet EPA standards. Get a good fuel controller like the Power Vision or the TTS with a good map, and a set of good pipes. Those two things will help.
I've decided that I'm not going to worry about every little noise. As long as the bike is running strong and there's not a sudden, noticeable change in any noises, I'm just going to enjoy the ride. After a couple years, I'm sure I'll be a lot better at recognizing and diagnosing all the different noises.
Funny thing is, even with all the problems and lousy service from HD, I wouldn't trade my SG for any other bike.