840 CI Cannot Diagnose Droan/Bump in 5th Gear at 1500 RPM

I have posted twice on this matter. I have searched everywhere for transmission issues, Giubo,/Flex Disc, Center Bearing, out of balance driveshaft, bad wheel bearings and none of that seems to match my symptoms. A reputable foreign car mechanic inspected and drove the car and does not think the problem is any of that - not in the drive shaft. He recommended to drop the trans pan and search for a clogged filter or more serious trans damage. My car makes a droaning noise under constant moderate acceleration at 1500 rpm - normal cruising speed. Let off the gas and the droan immediately disappears. If I increase acceleration I get a slight bump and the car stops droaning. You will see a slight movement in the tach needle when the car bumps. At a constant 1500 RPM the droan comes and goes in a consistent rhythem similar to a warble as you might have from a bad bearing and you will see the tach needle move in that same rythem. I really do not feel any sense the transmission is slipping - there is no revving of the engine, the car moves along and accelerates from all speeds just fine. Whatever the faulty part, it is impacting engine rpm, but again, I feel no sense of transmission slippage or engine revving other than the slight tach movement. The car operates and shifts normally. My only other matter is that when acclerating from a stop, there is a very moderate vibration - just not a smooth takeoff like my 540.

1. Can the center bearing be causing this even if there is no play in the drivshaft?
2. Can the Giubo be bad but show no visible signs of wear?
3. Can this be the transmission?
4. If it is the transmission, are there any repairs or adjustments (tightening of the bands, replace valve bodies, etc) that can be done to repair the fault, short of having to replace the transmission?

Any advice is greatly appreciated. I don't want to dive into this unknowing and start incurring unnecessary cost.

Thanks much.