Transmission noise!

[Gman97nm]

Hello! I am new to the forum! And fairly new to the automotive world! I have a 1987 GMC Sierra Classic and I've been using it as my daily for the past couple of years! It has a 700r4 overdrive transmission which has recently began to make a strange sound, it mainly happens while in overdrive and after being driven for a couple of minutes. The sound will go away if the vehicle is put in park/reverse. A friend of mine checked it out and we discovered the fluid was overfilled, we drained a bit of it and the noticed that the fluid was not being pulled from the stick when the truck was started. Any suggestions as to what it may be would be greatly appreciated! I will post a link to a video in which you can hear the noise at the beginning, and at around 20 seconds. Thanks in advance!

https://www.youtube.com/watch?v=r0lMc-je-Hs

 

[1987 GMC Jimmy]

Were you the last one to service the transmission, and if so, did you change the filter? I wonder if there's something wrong with the pump or the converter. I'll bet you can find out by hooking up a gauge and testing the trans pressures.

 

[Gman97nm]

The transmission filter was changed about 2 years ago at a shop of a family friend, but I will be sure to take your suggestion into consideration. Thanks!