NP-435/4th gear noise?

[Power By Ford 70 F250 4x4]

The transmission seems tight.It was an old firetruck.It has only 8000 original miles.It had a PTO hook up.It shifts good all gears sound normal til you get up to about 50 mph then you let off the gas and it makes a lot of loose gear sounding noise you step on it and it goes away let up it comes back.Any clues?

[cep62]

Did you check your U joints?

Also an old fire truck may have 8000 miles but it may have a lot of idle time .

Also these old trannys make more noise than modern ones , it might be normal ?

[eggman918]

The 435 in my truck had 700,000 miles on it when I replaced the 390 with the 4BT and was still in OK shape,it was a little clunkey on the 4th to 3rd downshift but was still quiet well as quite as ther ever are in all gears.I too would check U-joints and the carrier bearing first, then differential wear/backlash.