1989 Ford F350 Super Duty Diesel making unknown sound
 

My cousin drove his 350 the other day and all of the sudden he started hearing a noise from the bottom of the cab...It kinda sounds like Meracas (the mexican music instrument)... The sound starts when in about 3rd gear and just gets lounder from there.... He just had the carrier Bearing replaced and also the driveshaft....When he pushes the clutch in, the sound is still there, we took it to a mechanic and he couldnt give a straight answer of what it could be so we took the truck and now were trying to figure the problem out... So if anybody could help the woud be great.

Thanks