2001 750IL transmission buzzing on upshift

[ac680]

I have an impeccable 2001 750IL, 83K miles now.
As long as I have owned it, 5 years, and driven 5K miles, it always had this buzzing issue.
Fluid has been changed with the correct filter, level triple checked and that did not cure the noise.
Seems a little more pronounced when cold, slightly better when warm but still there.
It always happens during an upshift at low power, typically 1-2 , 2-3, at higher speeds I cannot hear in anymore.
No codes, not slipping. not buzzing during acceleration, until an upshift happens.
Also happens when reducing speed and a downshift happens mostly 2-1.
With brisk acceleration there is no buzzing noise at any shift, only with low power use like in the city slow moving
I suspect an issue with the pressure regulation mechanism, as it drops pressure during a shift to get a smooth action, it sounds like a cavitating pump.
My transmission shop tells me that raising the pressure slightly might resolve it.
Possibly a software update ?
Thanks.
Henri.

[BONDOSPECIAL]

I had the same buzzing sound on my 2000 750 and fixed it. The solution was installing a genuine Filtran filter. I also had a sucking sound for the first few minutes after starting this car on cold days. The solution for that was carefully redoing the fluid fill procedure. It is easy to have this transmission 1/2-1 qt underfilled even when you think you have filled it correctly. This transmission is extremely sensitive to fluid level for some reason.

The procedure I followed for filling was:
1. With the car off, fill the transmission until fluid started trickling out.
2. Start the engine. Add fluid until it starts trickling out again. Install the fill plug.
3. Turn AC on to raise idle speed. Step on the brakes, put the car in manual shift mode (shifter all the way to the left in drive), select each gear, pause briefly in each gear.
4. When transmission fluid temperature is above 30 degrees C but below 35 degrees C, monitored using a scan tool (not an infrared thermometer! Not accurate enough) remove the fill plug.
5. If transmission fluid is not trickling out of the fill hole now, add more fluid until it is.
6. Wait until transmission fluid temperature reaches 40 degrees C.
7. Allow time for excess transmission fluid to drop out, then reinstall fill plug. Torque to 100 NM

I found that an IR thermometer on the trans pan is not even close to accurate. I underfilled my transmission several times and had noise until I dug my scan tool out and monitored fluid temp that way.