Whats going on...

I have a 2000 elantra with a 5 speed manual, just turned 190,000...

Recently after about a 45 mile trip on the highway i noticed a groaning noise when getting off. The noise seems to start as a slow groan at about 30-35 and changes in pitch/frequency as speed increases. The noise will happen only based on vehicle speed and not engine RPM, and will happen in neutral as well as in gear. I am a mechanic, and have done just about every routine maintenance to the vehicle as required. Based on the symptoms of the noise, I checked for tightness in the wheel bearings, and inspected in the air anything related to forward movement. I replaced the LF wheel bearing about 15,000 miles ago, and figured now the RF one was due. After replacement of the RF wheel bearing, the noise is still there. I don't know if potentially the LF has failed again, or if there is some sort of output shaft bearing in the transmission that could be making noise. I'm just checking here to see if anyone else has experienced this and could point me in the right direction...