Help: Rear end making weird noise [NOT differential clunk]

[bosozoku.s14]

hi everyone,

the rear end of my 95 s14 is making weird knocking noise when i driving on bumpy surface on freeway...it almost sounds like something hitting the trunk.

the funny thing is it doesnt make that noise when i hit big bump on freeway...

its not differential clunk, because i have SPL subframe spacers installed, i know how it sounds like and i already used to it.

could that be bad axles? something wrong with my open differential?
please help me figure out this problem, since that noise is killing me and make me want to get rid of the car

any help will be highly appreciated...

I tried:
1. took out everything in the trunk - no luck
2. secure the spare tire - no luck
3. change differential fluid, although it doesn't seem like helping this issue, i thought i'd just give it a try. - no luck
4. i checked the trunk latch too - no luck