borinquen si

Hello. I own a 99 Honda civic si with 147,000 miles. A few months back I noticed a vibation between 30 and 45 mph. It doesn't matter what gear it is in and if I accelerate slow or hard. I've replaced both cv axles and the intermediate shaft I've had 3 different sets of wheels on the vehicle and still the same thing. I've changed brakes to include rotors. It doesn't happen when deceleration but I figured why not. They were the original brakes the car came with since 99. Ive also changed the driver side top motor mount and put polyeurathane inserts on all the rest of the mounts,otor and transmission. I'm really at the end of my rope here. I have no idea what else it could be other than the transmission itself or clutch. Any input would be very helpful. Thank you