Help! 2005 Civic over-revving after 2nd gear?

[Lance2020x]

Hey guys, I'm not exactly sure how to explain it, but my 2005 Honda Civic EX Coupe (5 Speed) is running fine, until I shift above 1st gear and get above about 25-30 mph, then it sounds almost like there's a second engine revving... I don't know how else to explain it, it sounds like it's over-revving but the RPM stays within normal levels, and when I go anything above 25-30 mph it SOUNDS like I'm still in 2nd gear, but everything else, RPM, etc. act exactly as they did a week ago (before this sound started). It almost sounds like suddenly there's a second engine running full force on top of the normal sounds the car made a week ago. I don't know how else to describe it.

 

I'm a little freaked out because I take good care of my car and I have no idea what this is. I just had the oil changed and they checked the fluids, and I personally checked the transmission fluid. I'm stumped!