86 scottsdale sm465 tranny noise

[8swbk10]

hey guys i just got my truck inspected and the first day on the road it was fine for about 15 minutes then all of a sudden when i let off the gas it makes a loud howling noise it only happens when i let off the gas but if i push in the clutch it goes away. i checked the clutch fluid and tranny fluid and theyre fine. can it be the clutch, pressure plate or throw out bearing? or maybe even the transfercase? im pretty sure the s clutch , pressure plate and t.o.b are stock. when i did a engine swap in 08 we did not change any of them because they looked great. i can't debunk what the noise is and i dont want to rip everything apart yet with out knowing exactly what it is any help would be greatly appreciated thanks