I have a 1998 model 2500 with a 350 and automatic transmission.
I don't drive it much and last summer I got a tapping noise in the engine.
I have only started it a few times since to analize the problem.
It has new oil and when I first start it I have no noise. As soon as the enginge warms up I get a thumping noise around 2200 RPM. The noise always gets worse as I increase the RPM. A quick acceleration always seems to cause a louder thumping. When the thumping occures and I let off the gas the noise stops right away. I get the same noise when driving it.
It doesn't sound like lifters. or a rod knock or wrist pin. It holds very good oil pressure.
I pulled the flywheel inspection cover as much as I could (4wd) and all bolts were tight. I could not actually inspect the flex plate.
When I was under the vehicle I could hear a light thumping but when on top of the engine it sounded loud at the rear and above the engine.
Could this be a main or blown intake gasket. It is not a hollow sound but more of a plastic mallet sound increasing with RPM but not on deceleration.
Any help is appreciated.

 

Can you locate the area that the noise is coming from, upper end (rocker arms etc..) lower end, (connecting rod)? Is there any end play in the harmonic balancer?

My first thought is that you have a connecting rod bearing that is worn or a piston that is slapping.

Also check to ensure that your anti-knock sensor is plugged up.

Have you put fresh gasoline in the tank recently?

A mechanics stethoscope can help you locate the area the sound is coming from as well as helping you hear the sound better. If you do not have a stethoscope then you can use a long screwdriver or a piece of wooden broom handle to help you hear the sound. Place the piece of handle/screwdriver on the engine and then place your ear on the other end and you can hear the noises inside the engine or whatever you want to listen to. Be careful around any moving parts because if the handle gets caught in the fan or belts serious damage to the vehicle or injury to yourself can occur.

 

The balancer is straight and no bad endplay. The noise is internal and more towards the back and higher part of the engine. I know the problem is not lifters, wrist pin or piston. As the RPM levels off the noise pretty much stops and always stops on deceleration. I'll spend the weekend with a good stethoscope and see what happens.