rumbling/vibration, please help

[Kristen_S14]

There's a vibration in my car that I notice on the highway. It doesn't shake much, but it's loud. It only does it when my foot is on the gas, and if I shift down to fourth it stops. It gets progressively worse as speed increases. I recently had a transmission rebuild and new motor mounts installed, but neither seemed to affect this particular problem much. Any ideas? I thought maybe CV joints but I don't know whether or not that would explain why it doesn't happen in 4th. Thanks!

[ELEMENT692121]

drive shaft .. sounds like you have a joint in your driveshaft thats seizing up when u load it up with a lower gear it changes the strain on the shaft .. ive seen this happen quite a few times before .. usually the noise starts around 40 or so and gets louder and louder the faster you go ..