Mysterious Rattle Vers. 18 Miata

Out and about yesterday. Come to a stop on an on-ramp (cause why would you go on an on-ramp?)  I hear a rattly noise. 

Not excessive, but enough to get my attention.  No other cars that close to me, nothing I saw on the road (although I could have missed something.)

The noise was akin to loose brake components, like pads without the proper retaining hardware, or dry/damaged CV joints clicking and popping as a car turns but quicker.

I visited local member jgrewe and stole his lift for an hour and change and we poked around, did the 60k nut and bolt check, looked at bushings, everything.  Didn't see anything obvious. Bolts were tight, bushings were good, complete bottom of the car was dry.  Wheels spin good and true, no bearing issues. 

No clue what the rattle could be.

Is there something peculiar to the ND that we may have missed?

Additional thought, likely not related but who knows, on coast down I get a kind of whmp-whmp-whmp as if a brake rotor were warped, but again this is coasting down, not braking.  Linked? Another issue?

Paging Keith. :) 

I was just thinking... When we were spinning the wheels checking for trueness I didn't look at the tire treads to see if any strange wear pattern was showing up. Maybe a tire starting to have a belt problem. The sound could always be there but its covered by road and engine noise at speed. Coasting down the engine noise drops enough for strange noise to be heard.

In reply to jgrewe :

I didn't see anything obvious. I'll poke at the rubber again.

Center caps loose in the wheels?

In reply to Pete. (l33t FS) :

I didn't check explicitly, but I wasn't gentle taking the wheels off at the shop the other day and didn't hear anything when I set them on the ground?

Watching. I put my stock rear brakes back on my '19 ND last weekend with new pads and rotors and now I have some obnoxious rattling/scraping noise coming from the rear right under acceleration and left turns.  It sounds like the wheel is rubbing on something but I can't find anything wrong, it's making me crazy. I'm going to take it all apart and reinstall it this week.  It's possible that I didn't align the little nub on the brake pad with the X pattern cutout on the piston. 

Wheel bearing maybe? It seems like modern wheel bearings fail in weird and varied ways. We had a low-mileage wheel bearing failure on our late-model Subaru, and it sounded like another small engine in the trunk, like a whirring/pulsing noise level scaling with vehicle speed. We could only hear it over 35 mph but less than 55 mph because road noise started to mask it. Running in gear on a 2-post lift confirmed which wheel, and replacement fixed it.