Malifore

While in even gears, 2nd and 4th mostly, at 3k RPMs the shifter head will vibrate more aggressively. Not insanely so but just enough to make a rattling sound. 1st and 3rd gear are not effected by this under the same conditions. Could this be as a simple as a shifter bushing or something more serious that's wearing out or possibly a lose bolt? What do you all think?