own a 93 Cadillac Seville with a 4.9 When I first start up the engine, or when I have the car in park and rev up the engine to 3000 RPM's, there seems to be a loud knocking sound that is coming from the rear side of the engine(when looking at the car's engine). Sounds like it could be the transmission, but not sure because it shifts fine when driving. Anyone have any ideas that can help me troubleshoot?

do this... when it's in park, take 1/4" driver and put in the belt tensioner square. (there's a place for a 1/4" driver to fit in so you can put on a serpentine belt) pull up on the tensioner and the belt will stop turning. see if that makes it quit thumping. if so, you have the common main bearing thump...nothing to worry about. let us know.

Loose the accessory drive belt while the engine is running to see if the noise goes away.


The main bearing thump that was mentioned is not too uncommon on older, high mileage 4.9's. It is harmless but is typically a very low, hollow thump noise that is not specific to one side of the engine or the other. It will disappear when the belt is removed or loosened so that is a good test for it.

If the noise is very loud it is probably not the main bearing thump described.

Is the noise more a harsh metallic "clack" that is rhythmic and occurs relatively slowly at idle compared to engine speed..??? If it is more of a clack and is on one side you may have a broken rocker arm pivot, rocker arm support bar or something like that in the valve train. That would make a noise that would be on one side of the engine or the other and would sound up high on the engine. It would be a very loud, clack-clack-clack that varies with engine RPM. Remove the PCV valve/clean air supply fittings from the valve covers and listen thru the holes to see which side it is on. Remove that valve cover and inspect the pivots and rocker arms. You can start the engine with the valve cover off and listen to each of the rocker arms/pivots with a piece of rubber hose like a stethascope to isolate the noise source. Put one end of the hose in your ear and move the other end around to see where the noise comes from.

LacSeville... tried putting a 1/4" driver into the belt tensioner square and it still makes the noise at start up. Doesn't make the noise at high RPM's anymore. Only at COLD start up. Think it could have something to do with oil pressure????



bbobynski... it is more like a harsh metallic clack but only last for about three or four clanks at Cold start up only. When the engine is shut off and then started back up (while the engine is still warm) there are no clacking noises what so ever. I asked another buddy of mine and he said that it could have something to do with oil pressure and my lifters,(whatever those are :hmm:) and he also said that my engine will probably be shot soon:rant2: . What do you guys think about his hypothesis??? Does it make any sense?

don't just put it in.... you have to make the tensioner swing up and loosen the whole drive belt. pull on the driver... the tensioner assy will swing up. you have to start the car, and while it's started, do this. you lifters would have to be leaking down alot while the car is parker, for your friends theory to hold water. i don't the lifters are going to be your problem.

Alright thanks for your help LacSeville, i'll let you know what i find out and see if your theory holds true, which i hope it does!

Which brand & weight of oil are you using?

High milage(since my car has 122k miles), Pennzoil 10w-30