What's causing this annoying droning noise in my cab?

[Skunksmash]

There is some weird droning noise in my cab. Please listen to these sound bites in this zip file, and tell me if you know what is causing this noise in my 1987. I sound deadened the vehicle, and now you can't really hear anything outside the cab. But everything inside the cab is way worse now, since you don't have all that other road noise and such to mask it. It sounds kind of like an old blower motor or something, but that can't be it cause I unplugged the blower motor and it's still there. I even tested to make sure the blower motor wasn't working by trying to use the controls. So we can rule that out I guess. It's kinda like... how you might think of a weedeater would sound, if it were on an video game or something. Not really like a real weedeater, but sort of. And much quieter than a real weedeater, of course. Maybe it sounds like an ever so slight vibration, that you can not feel, only hear the effects. Anyway, here is the sound .zip file. You may have to download it and open it (or right click and hit extract, then open it) or something like that. But your windows media player should have no problem playing it. You also may have to turn your speakers up a little, to hear it.

[Skunksmash]

If it helps, no amp or sub or anything, just some fairly cheap aftermarket coax speakers. They're not crap or anything, they're just not the best. I'm sure if you went to the local car audio shop they would charge you like $80 for the rears and $90 for the fronts. But I bought them online, so in all likelihood they were much cheaper. I never checked what the MSRP was. So all in all, I don't think they are they types that would generate the sound. Also, I put my ear right up to all four of them. And it just stays faint. The only place you can put your head up to, and have it get louder, is under the middle of the dash. It has something to do with the engine, though. It changes pitch and tune as the RPM's go up. That's what's so confusing. As soon as you hear that, you think "that's gotta be alternator feedback". But you just can't hear anything when you put your ear practically on the speaker. Might end up having to take it somewhere that is good at tracking down automotive enigmas.

[Irish_Alley]

in the sound clip are you driving the truck or even revving the engine

[VileZambonie]

Did you turn off the radio? Get under the dash where it's loudest with a stethoscope... if it's coming from the engine compartment loudest then make sure nothing is rubbing or hitting. Do you have a.i.r.?

[AdrianR]

x2 VileZambonie

Try to isolate the source - it sounds suspiciously like residual engine noise...perhaps there's a sympathetic vibration going on somewhere as a result of the sound deadening material.

[83k1500]

Does it do it only while moving or does it do it while sitting too?
Since you mentioned it is RPM sensitive have you got under the truck and listened?
Have you removed the driveshaft and looked for wear on the output yoke (the part that slides into trans) or checked for play or stiffness in the u joint caps?
Also what kind of play is in the steering box if any? A slushy box can do weird things. How much does the steering wheel turn before the front wheels turn? if it is anymore than an an inch to an inch and a quarter you need to tighten it up. It could be pump feedback being echoed in the shaft.

It does not sound like feedback to me, that is higher pitched and very distinctive. That sound is mechanical.

[gbskunk2006]

... It's kinda like... how you might think of a weedeater would sound, if it were on an video game or something. Not really like a real weedeater, but sort of. And much quieter than a real weedeater, of course. ...

Hey Skunksmash,

Funny, I've noticed the same kind of sound and was going to post something soon but you beat me to it   I also sound proofed my entire cab so I know it's not road noise.  During my troubleshooting, if I turn off my stereo, I think the problem is coming from the speedometer and/or cable?

Try sticking your ear close to the speedometer and see if come to the same conclusion.

Thx in advance.

[Grover 1]

My constant noise goes away when I leave my wife at home!

[Skunksmash]

Ok, sorry it's taken me so long to reply back. Been real busy and stuff. Well to answer a few of the questions direct at me:

In the sound clip, I was not driving or revving. It was sitting in park, at idle. Radio is completely off in the clip. The truck has all it's stock emissions, so it's my assumption that it does have the A.I.R still. Haven't really looked for the smog pump, but it passes the required emissions test with flying colors, (barely puts out half the emissions it could, and still pass) so I think it's safe to assume its all still there. You can hear the sound any time the truck is running, either in park, or going down the road. I have not had a chance to use a stethoscope yet, should have one around here somewhere, the GF is a nurse. I'll just borrow her $200 stethoscope lol.   But naw, I better not do that.   I'll use the old cheapie.

The guys over at FullSizeChevy think that it's the fuel lines vibrating against the firewall (or transmission tunnel), since I have added one of the throttle body spacers from CFM tech. Also I have had to have the tranny rebuilt in the past, so maybe they did not secure the fuel lines as well as they should have? I'll check all this stuff out as soon as I can and report back. The more I ask around, the more people seem to be coming up with the same problem. The only thing I would like to ask now, is does anyone have a pic of where the fuel lines run, on an '87? I think I may be out of luck, (when it comes to being able to fix the problem) if they run up against, or inside the tranny tunnel. As I may not be able to get to where they are making contact with the cab. If that's actually the problem.

[stinkyscooter]

I had a simaler noise in my 85 k10 and it turned out to by a worn out buzzer that was wired to my drivers seat belt.The noise came from the center of the dash. I cut the wire of the bottom of my seat belt and it went away. Hope that helps.

[Skunksmash]

 Ok guys, sorry about the extremely late update. I have just been very busy with work and school, and have not even had time to work on the truck lately.

So yeah, it's the fuel lines. Even just wiggling them around a little bit has drastically reduced the droning noise. It's still there, but this gives me the confidence I need to get the problem permanently fixed. I wiggled and jiggled them a bit, up by the throttle body and intake. But now my question is this: What should I do to actually fix the problem? Due to the throttle body spacer, (which I guess actually caused the whole problem), the retaining bracket that once held the fuel lines is now out of reach. They can't be put back in it. So I am thinking maybe a solid rubber bushing of some kind?

The only problem I have with sticking a rubber bushing in between the lines and the firewall/transmission tunnel is... will it somehow hurt the lines in any way? They are already being stretched a little bit in order to accommodate the throttle body spacer. If I make them bend a little more, could I be looking at a possible fuel leak, or weakend lines? I mean the truck is from 1987... so it's old. Best not to push things that were never meant to bend... too far.

[beastie_3]

You can take some rubber hose with a inner diameter the same as the outer diameter as the fuel line, cut a slit down the length of it and wrap the fuel line with it. Just like convoluted tubing for wires.

[chevyman72]

Fuel lines are supposed to run in the frame rail to begin with.

[VileZambonie]

Fuel lines are supposed to run in the frame rail to begin with.

[Irish_Alley]

Lol silly.