I have a rattle when riding on bumpy ground and i can' for the life of my figure out where its coming from..
Seatpost bolt is tight, the bb and cranks dont have any play other than chain looseness
Its fixed so no deraileurs, front brake is tight, handlebars don't move.
Only think i can think of is the chains rattling against something?
Any number of things. Valve nuts loose rattling on rim, or no valve nuts causing valve to rattle against the rim. Headset loose, pedal bearings worn, loose spokes, loose toe clips, bottle cage nuts, chainring bolts...
Not sure what you mean by front brake tight. Check for loose lever or caliper nut not tight in the forks.