Disc brake rattle question - YY250T

[Shadowfire]

I recently purchased a 2nd hand 2014 Jonway YY250T scooter with 450 miles which didn't turn over. Here's the bucket list so far:

* recharged battery (engine now spins but still won't startt)
* stripped off the seat and most of the plastics
* cleaned out the carb (it fired up and ran). replaced a ton of clamps on the carb.
* changed the Oil and cleaned the screen (old oil was black, screen was actually clean)
* changed the antifreeze (old stuff was pale yellow) with 50/50 prestone premix
* changed the tranny fluid (looked like it had a lot of metal particles suspended in it)
* restart the bike to bleed the heating system... leaking antifreeze @ 5000 rpm. Coming out of the water pump feed into the cylinder at the connection. Clamp was broken, someone had put Teflon tape (!) inside the connection instead of replacing the clamp- replaced both the feed/return hose clamps here, since I don't want problems.
* exhaust was leaking at the cylinder head connection and muffler was starting to show rust. ordered new gaskets, sanded down and repainted the muffler with high temp primer/paint, ordered stainless steel bolts/lock washers/washers for the heat shield
* waiting for a USB charger port and some electronic-safe silicone. NONE of the connectors I've seen are weatherproof. Going to silicone, shrinkwrap, and zip tie it all together.
* trip meter reset button is impossibly hard to activate. gotta get in there and see what's going on, hopefully some lithium grease is all that's needed.
* ordered a top end kit (piston/cylinder/rings/gaskets) in case there's no power or if the next oil change is contaminated.
* surprisingly, the existing fuel line was DOT certified, so I didn't end up replacing it.

Anyways, I haven't actually driven it around yet, but when I revved it up (prior to removing the muffler) on the center stand to about 6000 rpm I can hear some sort of metal rattling which seems to be coming from the rear disc brake. Now the caliper itself seems to be bolted on good and tight, and the disc itself has no loose screws. I seem to be able to rock it side to side a tiny bit, and the front caliper has the same amount of side-to-side play. The reason I'm asking is, I'm not sure if this is OK or not. I'm leaning towards it just being engine vibration rattling the calipers around. They are definitely on there good and don't budge at all if I try and move them up/down (like they would be forced to when braking). I do not hear any rattling below 6000 rpm. Is the rattling a cause for concern? Or am I mis-diagnosing this?

Anyways, this bike has been a real eye opener. My other bike, a 50CC Kymco Cobra Racer, has given me 12 years of trouble-free service, with only tires, batteries, valve stems, and belts being replaced (along with a carb cleaning when it sat too long).