Can't find source of Whistling

[LTZ C20]

I have 79' Chevy Cheyenne C20, SB350. Over the past two months I have rebuilt the entire top end, rebuilt the carburetor, rebuilt the distributor, put on a new intake, completely reset the timing, firing order and idle and mixture. Now the truck is running great while your actually rolling. When it is cold it runs great until about 15 minutes later, usually my drive to school from home. At that point it idles rough but doesn't die. If I give it a rev, sometimes it idles normal again or continues to lope. And finally now that I have done all of this, when its idling normally, it has this whistling and I can't figure out why or where it's coming from., it doesn't do it while accelerating or driving at a constant speed. I have eliminated everything I can think of. I think it might be a vacuum leak and that's whats causing the rough idle when it warms up. Anything you can offer or suggest is appreciated. I have reached my mental know how, not very good with troubleshooting. Thanks ya'll.
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[Jason S]

Can you hear the whistling more in the cab or in the engine compartment?

If it's in the cab and it is/was originally air conditioned, sometimes the vacuum diaphragms or vacuum portion of the control panel switch can develop a small leak that will whistle.

If it is coming more from under the hood, then you have to verify all the vacuum hoses are connected and/or unused vacuum ports on the carburetor are capped and that the vacuum hoses don't have any cracks in them.

To eliminate the vacuum hoses as a culprit, verify that the following hoses are firmly connected: vacuum hose to the distributor, vacuum hose to the PCV valve, vacuum hose on carburetor for the choke pull (if automatic choke), vacuum hose to the vacuum reservoir on passenger side firewall (A/C truck), vacuum hose to the power brake booster and vacuum hose to the transmission (if automatic transmission). 

Also check any vacuum fittings that screw into the intake and back of the carburetor. You can use teflon tape on the threaded vacuum fittings to ensure they make a good, tight seal.

If the vacuum hoses aren't the problem, then you might try the old trick of spraying carb cleaner or WD-40 where the carburetor mounts to the intake. While the engine is running and warm, spray the carb cleaner around the base of the carburetor.  If the engine speed changes, then you found a vacuum leak. The same thing can be done where the intake manifold meets the head.

There are other things to check, but first I'd ensure your vacuum hoses are all properly connected.  Hope this helps.
   

[LTZ C20]

Thanks, I have done all of that including the carb cleaner spray trick. I can't find it anywhere. It's under the hood and its loud.

[bake74]

      This is not what you are going to want to hear, but the things you described do sound like a vacuum leak.  Now you said you tried the carb cleaner spray trick, did you check it everywhere there is a hose and around the engine you worked on and around the carb ?
     Next is process of elimination, start with one hose at a time and check it twice, both ends (you might have loosened the other end of a hose while working on engine and not realized it).  But I would suggest you start with the hoses and parts (intake and carb specifically) that you actually moved/messed with while rebuilding the top end of your engine. 
     As far as intake and carb. make sure the gaskets are there (maybe a piece is missing and you didn't realize it), and you did make sure the intake was flat against the heads when you installed it ? 
     

[LTZ C20]

Well I will redo a spray check on everything and see what happens, yes the intake was flat and everything was working. It just started after I rebuilt the distributor. I'm sure my gaskets are ok, but I will check again. Thanks.

[Jason S]

If it started after you rebuilt the distributor, have you tried working back from there?

I'm just throwing out some more ideas to consider:  Remove the hose to the distributor from the carburetor and cap the port on the carb to see if the whistling stops.  If so, then you either need a new vacuum line or a new vacuum advance unit.

Same thing with the remainder of the vacuum lines, remove and cap the ports at the intake or carburetor... 

If you remove all the vacuum lines, the vacuum ports are capped off and it still whistles, then you have a vacuum leak at the carb or at the intake to head gasket location. 

[bd]

So, here's a little trick that might help you locate the source of the whistle (and many other troublesome noises that are difficult to pinpoint).  Get a 3' length of 1-1/2" ID rubber radiator type hose.  While the "whistle" is "whistling," with one end of the hose to your ear, move the other end around the engine compartment until you recognize the "tune."  The hose filters out much of the background noise and amplifies the sound entering the open end.  I've pinpointed many an impossible noise this way, even when stethoscopes failed.

Curious.  When you O/H the distributor, did you remove the drive gear from the shaft and lube the bushings?  What carb do you have?

[LTZ C20]

At Jason, I have actually put a new vacuum advance and new hose on the distributor. I believe all vacuum lines are in good condition, they all flex well and aren't crispy. At BD, holy crap that's a great idea! I am so going to do that as soon as I have the chance! Thanks a bunch! Almost forgot, I did pull the drive gear yes, and I have a Edelbrock Performer 600 50state electric choke.

[bd]

The distributor gear has a 1/8" dimple on one side adjacent to the roll pin hole.  The gear should be installed so the dimple is pointing in the same direction as the rotor is pointing.  If the gear is installed 180 out, the rotor will be pointing between cap terminals when the spark fires instead of aligned with a cap terminal.  This can cause frustrating misfire, stumble, etc, and may not display consistent symptoms.   > It's one more thing to consider if everything else fails to resolve your drivability complaint.

[LTZ C20]

Thanks, I installed the gear correctly and I knew about the dimple. I tried the radiator hose trick, just with a straight shop vac tube cuse I didn't have radiator hosing. That led me to the EGR which I had suspected but wasn't sure about. Now I believe I need to replace those EGR gaskets and that should eliminate the problem. I believe the whistling is causing the rough idle after warming up. Aside from the whistle, the truck runs great!

[bd]

Great!  Let us know how it turns out.   

[LTZ C20]

Will do.

[LTZ C20]

Ok guys. So I replaced the gaskets that I was talking about, the whistling has reduced some. I have FINALLY figured out that the whistling is coming from inside my carburetor. It's coming from the butterflies. Now the carb is a couple years old and worn bout still works good. Do any of you know how to stop the whistling permanently? If I can't fix it, I have to get a new carb and I don't have the money for a new one plus it's more difficult to find a smog legal carb here for Cali. I greatly appreciate all the help.

[bd]

I know you've already sprayed for vacuum leaks, but try spraying directly at the end of the throttle shaft where the shaft enters the base.  And, try spraying all the way around the carb directly at the base gasket.  Be very specific.  Also, is the whistle dampened any at all by installing the air cleaner?

[LTZ C20]

I have done the directly at the base spray, nothin happens. And no not really, it does change when you pull the top butterfly closed though, sputters. Then you let go and it goes back to normal. The truck runs really good. It just whistles like a no tomorrow! haha