I have noticed a slight metal sounding jingle on my nimbus eclispe uni. It only occurs when the pedal hits the ground. I took everything apart and studied the parts and the jingle remained after the pedals, cranks, spacers and frame was off the wheel set. My theory is that there is a little play or wiggle room between the inner and outer races of the bearings themselves. That play would cause jingle. Has anyone else had this issue?
Is it audible, or can you just feel it being loose?
If it’s audible, check the allen bolts on the bottom of the saddle, often one loose bolt washer will jingle. 
. . . adorably at times
If it isn’t that and you suspect bad bearings, remove the bearing holders and see if there is any play back and forth with the bearings. with the wheel completely out.
A creak will often come from the bearings in the pedals if you have plastic pedals, but not really a jingle.
bolt by bolt until you find it is the way to go. Something is loose there.
Wiggle room in bearings
Yes, I took off everything, frame, pedals, cranks. I can actually feel the wiglw in the bearings themselves.
New ones are cheap enough. It’s hard to find a bearing puller that will fit it, but most bike shops will do it for free or very cheap.
Must ride pretty hard. I haven’t toasted any bearings yet, even doing Muni.