99 tahoe cabin noise - sounds like vacuum

[spicewood1]

I have recently bought another 99 Tahoe - it had a bad engine so the motor has just been replaced. Now I am hearing an audible sucking sound from behind or under the dash when the radio and ventilation systems are all off. Truly sounds like an old school Qjet with the air cleaner off for those who remember. It is definitely engine manifold vacuum related as it is loudest at idle and goes away with a blip of the throttle, then returns. Engine runs fine. Removing serpentine belt or plugging booster hose have no affect on the sound. I have sprayed ether all over the intake and fittings with no change in idle. It is not the radio cooling fan either. I have had 2 other 99's and this is the only one making the noise. Could it be something overlooked in the engine swap? What could make this noise? Thanks,
Paul

 

[718frankb]

shit i've been chasing the same noise since i did my headers .im about to rip my dash off to find it

 

[retorq]

That sucks ... get it, vacuum ... sucks. LOL

CAI maybe??

 

[drowndindreams]

at what RPM does your truck idle at? if its higher then lets say 7 or 8 hundred then you probably have a vacuum leak near the firewall.

 

[95TwinTT]

It’s been a while since I was inside the dash, but isn’t there vacuum lines that control the heater and air con, vent doors, that run into the temp control module?

 

[drowndindreams]

It’s been a while since I was inside the dash, but isn’t there vacuum lines that control the heater and air con, vent doors, that run into the temp control module?

if you mean the HVAC controls then no, it's just a whole bunch of plug and play wires behind that.

 

[spicewood1]

It idles at about 475 rpm, just a tick under the 500 mark. I'd love to bump it up just a hair because the engine resonance at exactly that rpm makes my side mirror rattle When I adjust the butterfly screw-stop the computer compensates with the IAC to bring it back down.
I see only a couple vac lines off the intake other than the booster line, disconnecting and plugging them does not help either. Could it be I am hearing plain air intake noise through all the abs plastic ductwork? Thought the Vortec was supposed to silence all that.
I considered the HVAC operators and valves inside the dash, but checked all the functions and they all work fine. I bad leak there would probably render the system inoperable.

 

[osukellen]

Man, I am hearing the same noise in my '99. I just put in the upgraded spider injectors about 2 months ago, and immediately heard it. Maybe I am just now hearing it because it was idling a little rough before??? Mine now idles about 400-450rpms, and if I barely bump the gas (up to about 600) the noise will stop. There are no vacuum leaks, at least that I can find. Could the distributor make this type of noise? It sounds like it is dead center just in front of the firewall.

 

[718frankb]

i was thinking it could be the distributor too i was leaning on it when changing the wires.little shit like this bothers me!

 

[osukellen]

If it was a vacuum leak wouldn't it be idling a little rough? Mine has never run better since installing the injectors, and has absolutely zero miss while idling or otherwise. What would I be looking for if I pull my d-cap?