First thing I'd do is try and tighten the nuts holding the 3rd member on. Could be loose and leaking. If not, you'll have to pull the 3rd to reseal it. Just use some black RTV to seal it up.

Check your fluid level, you could be low. If your too low, you could have damaged something. Put the truck on jack stands and put it in gear. See exactly where the sound is coming from.

88 4R, 350 V8, R150 5 speed Sold <...uot; />
97 4R, stock, daily driver
98 Sienna, kid and dog hauler, wife's ride