Well, I have been chasing a drivetrain droning sound for a while now and would like to know if anyone has any ideas that might help me find this annoyingly persistent problem.

I have a rumbling/droning noise in the car at certain speeds. It is a very low frequency and you can feel it in the back of your eyeballs....it undulates a little bit (rises and falls in strength slightly). The rearview mirror is vibrating like I have a set of sub-sub-woofers on at low volume...everything is fuzzy behind me.

It starts around 60 km/hr and continues to be noticable as you speed up and continues until around 80 km/hr when it starts to fade away after that.
On the highway around 100-120km/hr I cannot really tell if it is there or not....much less noticable.
Between 0-60km/hr everything sounds fine.

It is not related to engine speed only road speed.
It does not matter what gear I am in.
It is present if the clutch is in or out.
It does not change under load.
It is just always there through this speed range.

It seems to be located somewhere from the middle of the car forward...not so much in the rear end.

Here is what has been fixed/replaced which might help with these symptoms :
1.Transmission mount - approx 1.5 years ago and still looks fine
2.Front right wheel bearing - approx 8 months ago
3.Driveshaft center bearing and mount - 1 month ago (helped a bit)...also had them check the u-joints while the shaft was off and they said all is okay
4.Front left wheel bearing - last weekend (helped a bit since the bearing was adding some noise)

The tailshaft is not leaking and does not have any play.
The motor mounts look okay...my fan is centered and the oil pan has about 1/4"-1/2" clearance. Like I say it does not change under load (when the engine is torqueing).
The exhaust is not touching in any spots.

Is the driveshaft out of balance or are the u-joint angles incorrect? These are the more obvious things that I can think of right now.

If anyone can help it would be great. This drone/rumble has been around for about two years and I would love kick it.
I had the top end re-done after a broken valve spring, rebuilt the turbo and found a busted knock sensor only 3 months ago so everything else is running great now ... for a car with 250,000 km on it.

I would love to isolate this problem and do away with it.

Thanks in advance.