Happens in mine too. Exact conditions.... 50-62mph slight to half throttle in fifth gear...... I think it's the rear diff making some racket. But it's never gotten worse, or bled into any other gears/mph so I don't worry about it. Normal.

 

It's the t-case and it should never leave you stranded. 1st way to fix it, "turn the volume up". 2nd, "when you can't handle it anymore go get it fixed". Mine does the exact same thing. It's done it forever. I talked to a couple big evo guys\tuners in the community, I'm not going to quote anyone without their permission. You're fine. To put it this way.. I have a fully built IX engine. I'm currently sending my trans off to shep trans, pulling my rear diff off to put it together correctly as Mitsubishi should have in the first place, and I'm not touching my front dif because it's not going to break.