When it's fully warmed up and driving for at least 15-20 min, there's a serious metal to metal type rattling in the trans tunnel (almost directly under the shifter, you can feel it on either side of the tunnel) that varies with engine speed when hitting the gas at low rpms. Sounds almost like exhaust rattling against something but it's not that...

trans mount? Any other ideas?

 

U-joint(s)?

 

Doubtful as I had the whole drivetrain inspected -- whatever it is, is making direct metal contact with the chassis in or near the trans tunnel, enough so you can feel it in the seat/floorboards. Like the engine suddenly gets 10x louder. My first gut feeling was the exhaust vibrating against something but it's free and clear. It's to strong of a vibration to be a loose heat shield.

Is the trans mount a common failure? I don't recall seeing threads about it..

 

Sometime a u-joint will be loose only when the shaft is in a certain orientation. Might be worth a second look, as there really are not too many other possibilities down there. Happy hunting.

 

This is what I was going to say. It happened to me when I owned my 85 RX7.

 

it happens more when cornering to the left while accelerating under 2krpm. I have experienced U-joint issues in my G and it only happened during decel. It didn't feel like metal to metal against the trans tunnel/floor. It was more like shaking due to all the isolation of the drive train from the chassis.

But I'll double check the drive shaft, thanks!