Just bought a 01 Tahoe seemed to run quiet at first then after a while when u first start it it almost sounded like a diesel. Now a couple weeks later when you first start it there seems to be sort of a knock until u put it in gear and drive it a very short distance. Then it stops but it only seems to happen on the first start of the day or when the engine is set for awhile and the engine is cold.
Answer: If the knocking noise in your engine only is there when the engine is cold, this could be several problems.
The engine could have what is called piston slap. This is one or more pistons hitting the cylinder wall in such a way that it causes a knock. The piston can be loose on it's pin, the rings could be worn or a piston skirt is worn. As the engine warms up, the looses goes away because everything expands with heat.Another possibility is a cracked flywheel. The same thing happens here that as it warms up it expands and the crack stops making a knock noise. Either way, you are going to have to have a mechanic listen to it to decide what is wrong.