strange thumping/tapping feeling.

[bosozokujin]

Yo guys, I have searched and searched cant seem to find my specific issues, soo Ill shoot here. I have an s14 zenki with an sr swap.. I noticed when I got on the highway last night i went up to 70mph and right around that 70 or a little above. while my right arm in resting on the arm rest in the center I could feel like some tapping or thumping type of thing going on.. when i let off the gas and started falling below 70mph it went away. back to 70 again and it came back.. it feels like something is hitting something i know thats vague but cant think of much more, i looked underneath the vehicle and nothing going on.. no leaks no anything. my diff has fluid. anyone have a clue? btw car doesnt not shake or anything things are as usual only i can feel this bumping for the most part when my arm is sitting on the armrest compartment cover. not a comfortable feeling .