I did the plenum gasket repair about two months ago and since then the engine has had a peculiar noise that is driving me nuts. I can't figure out what it is. The truck is a '99 1500 with 5.9 L engine. The sound is really only audible from inside the cab. We've tried locating it under the hood with the engine running, but the best we can tell is it's coming from the top rear of the engine, maybe near the distributor. From inside the cab, it is a buzzing noise, almost like a loose bolt rattling and it sounds like it's coming from inside the dash on the passenger's side. At idle, you can't hear it. When you accelerate, the buzzing begins and when you let off the gas, it quits. It's definitely tied to the accelerator somehow.

My latest theory is that it's coming from the cooling system. But no leaks, oil in the pan is clean, no sign of cooling system problems, in fact, the engine has never run better. Seems to be more prevalent when the temp is colder and the engine is cold. We had some really warm days a week ago and the noise went away temporarily. Could it be air in the cooling system? Is there a way to bleed it? I have also noticed that some days when I start the engine, it sounds like a rush of water behind the dash.

Any theories????[sm=headbang.gif]

 

When replacing the intake manifold it is very easy to not get a good seal at the back. One possibility is that the buzzing noise is a vacuum leak in this rear area.

If the sound changes as you go from idle (high vacuum) to more open throttle (low vacuum) that might be a hint to look deeper into this.

 

if you have it in park and you step on the gas, can you hear it then?

 

In park, you don't hear the noise. The engine has to be under some sort of load, even a minor one.

 

Check the air filterbox or gasket. I didnot get mine on the TB quite right, it was off just a little bitand it was making a funny sound. Just a thought.

 

Could be a loose heat shield or one thats too close to the parts its protecting

 

I get that rush of water behind the dash sometimes when starting up also and i can't figure out what it is for the life of me

 

i always thought the water behind the dash was just coolant going into the heater core

 

It is.

 

It sounds exactly like a buzzing heat shield, or something like it. I just made a new gasket for the air box connection to the TB and we've got that good and tight. I thought maybe that was it, but no luck. And I have to edit my response. I did hear it in neutral today. Tap on the gas and the buzzing picks up. Let off the gas and it quits at idle. Correct me if I'm wrong, but a vacuum leak would be between the manifold and the plenum pan? I'm pretty sure I got a good seal there. Would there also be a vacuum leak between the plenum pan and the block? i.e. in the lifter valley?