I just replaced the moter in my 1975 C60 dump truck. I got the moter out of a 81 chevy pickup. The moter from the truck ran and sounded fine. Swapped the altornator, water pump, power steering pump and distributor off the old dump truck moter. otherwise didnt touch the new moter. It is installed runs great but now there is a hollow knocking sound coming from the front of the moter. What could this be from?