My 1991 B2600i had a squeaking noise, inside the cab......took months of driving to finally figure it out. It was the dash mounting bolt on the driver's side of the dash, under the plastic cover......the bolt was loose, and the metal tab on the dash was rubbing the metal mounting area, whenever you went over rough terrain, or speed bumps, etc. I tightened the bolt down and no more squeaking after that.
