AW71 Automatic transmission problems.

[LarRy]

Hi,
I have an AW71 automatic transmission in my Volvo 740DL, 1986. Was driving rather hard in the Italian highways, 90mph, heavy loaded, long uphill and hot days. When increasing speed the automatic transmission stepped down normally from 4th to 3rd gear, but when returning back to 4th gear I heard odd sounds from the gearbox, sounding more ore less like an electrical window motor. This sound came back every time when the a gear shift took place and continued until the rpm of the motor was reduced to less than 3000, at 3000 rpm it instantly dissapeared. Otherwise the funbction of the gearbox is perfect. Is this sound coming if the gearbox oil is overheated, or is it something else?