I have a 1996 XJ which I swapped in an ax15 in November. Recently it has started making a light marble sounding noise, for lack of better wording, between 47 and 53 mph regardless of gear or engine RPM. It is influenced by engine load such as going up inclines (noise gets louder) or driving down a hill (noise almost goes away). It is also louder when the transmission is colder, but once warmed up it is quieter. I'm curious as to what would be causing this.
Note: When I did swap the transmission, I replaced the clutch, flywheel, and fluid, but the kit provided the wrong size throw out bearing, so that was not replaced and is probably in need of it, which I am planning to do when the weather is better. Could this be a cause of the noise?

 

Do you notice a change in sound if you apply light pressure to the clutch while at speed? Also at speed quickly fully depress and hold the clutch down to see if the noise stops completely. Rev match your rpms when releasing clutch.

 

When the clutch is lightly and fully depressed, the noise goes away. It seems that the noise only happens when the transmission is engaged and in gear, regardless of gear number.

 

While running at highway speed of you lightly depress the clutch without disengaging the engine from the trans but the sound changes or goes away completely that would sound like a TOB to me. I have had this issue happen in a 90 ram, 97 neon 98 neon 05 srt neon. I have been right in diagnosing all 4 failures with this method all 4 times. The 90 ram you could hear at idle and just by testing the weight of your foot on the clutch pedal it would completely stop.

 

Thank you. I definitely need to get around to replacing it.