2 noises I’m trying to identify...

[Andrews98]

Ok so the vehicle is a 98 2-door Tahoe LT 5.7L 4L60 Z71 6” lift on 35” tires (there that’s out of the way now) I purchased it about 7 months and am working the “bugs” out of it slowly but surely. Ok so....

Noise #1
There is a noise that can only be heard inside the cab when the engine is idling. I hear it behind the dash/firewall/transmission tunnel. Earlier on I did find the fuel lines rubbing the heat shield so that was taken care of. It sounds like what a water pipe in your house that has water running through it. Not the drain but the pressure side (has that low pitch hiss). It doesn’t matter what gear, what temperature, etc. it always sounds the same. It doesn’t increase or get louder with engine speed or vehicle speed. After it comes out of idle it goes away...or maybe it stays and gets drowned out by engine/fan/ambient noise, not sure.

Noise #2
So there is a low pitch hum/vibration when I let off accelerator either coasting or slowing down down. It feels like it’s the transfer case because I feel it right under my seat. I have since tore it down and has new bearings etc. The noise can only be heard from about 45mph and up. I’ve replaced ujoints on both drive shafts and I’ve replaced the slip yoke on the rear one with the output shaft bushing and new seal. My drivers side CV shaft has play in it but I changed that bearing and seal also. I’ve even thought about an exhaust leak of some sort because it does have that gurgling misfiring sound when I let off the accelerator. I’ve done a whole bunch of troubleshooting so far, and there is a thing or two I’d like to try but for now, I’d love to hear any opinions y’all might have. Thanks y’all.

 

[Motorgod0]

I noticed no one has replied to this and I don’t know if you’ve figured it out yet. I’m new to this site as well and have gotten a lot of help. So I’m going to try and pay it forward. I am no expert by any means so take it for what it’s worth.
#1: maybe a vacuum leak. Check the lines and replace as necessary. or something as simple as sticking blend doors. Goes away when it frees up. Try different fan speeds, on and off etc. a couple times to recreate the sound. If there is a light “thud” just before the hissing stops then that’s it.

#2: I don’t know what tread pattern you have on the 35’s but heavy/thick tread can cause that sound and sensation. And it’s exaggerated if the balance is off. I had a similar sound and sensation when running 35’s all terrains.

 

[evilunclegrimace]

Put the truck on jack stands with only the rear wheel off of the ground,Make sure that the truck is in 2WD and see if you can duplicate the noise. CAUTION!!! DO NOT RUN the truck excessively fast while on the stands.

If you have an open diff and the speedo reads 45 mph and only one tire is turning it will be turning at twice the indicated speed!!!! If you don't hear the noise then put the truck on four stands and try again, This will at least help determine if the problem is in the front or rear

 

[Andrews98]

Thanks guys for the replies. I do have the G80 limited slip. I have ruled out the A/C system (only because I finally fixed everything on it lol). On the way to work, I got up to about 60mph, let off the gas and then shut the engine off in neutral....same exact noise as before. So I now know its not engine but probably drivetrain. The tires have been balanced twice and I think they made the same noise when it had the factory size tires on it as well. Ujoints have been changed twice all 4 joints front and rear shafts. I have also had the transfer case out and replaced all the bearings for good measure and put the newer pump body housing in. Fortunately, there was minimal pump wear on the case. I have suspected my cv shafts because the driver side stubby axle had some wobble to it so I replaced the bearing and seal and it still wobbles. I have checked all the simple stuff I can think of. Ive tapped on everything with a rubber mallet checking for anything loose. Everything sounds solid unless Im not whacking on it hard enough. I have put it on jacks and ran it like that, and didn't really see or hear anything obvious, maybe the driveline needs to be loaded for the noise to be heard. I thought maybe torque converter because of all the symptoms or maybe loose flex plate bolts. Haven't really looked too hard at those yet, but from what others have said, I can't say that the noise is the same. I don't know, this has been stumping me every since I have owned it since last May.