OK so I'm trying to get to the bottom of a very annoying sound coming from the dash of my blazer when in gear, and sometimes just in park. Here's what it sounds like: It sounds a little bit like an old small block chevy that has it's distributor timing set too far an advance and it's pinging, though not metallic sounding. The sound is most prevalent while accelerating, though it can sometimes be heard while sitting idle in park or neutral if you listen carefully. The sound completely goes away, in reverse, and sometimes can't be heard while just sitting in park/neutral (almost like a soft tapping). The HVAC has no effect on the sound.

Nothing sounds out of the ordinary while revving the engine with the hood open in the driveway, nor does anything sound out of place driving with the windows down. You really need to have the windows up to hear it, and it's annoying, and I'm baffled. I've crawled under the blazer with it in idle and I cannot hear the noise, but I can hear it in the cabin with the windows up.

When the sound started to occur: I've never heard this sound until I gave the blazer to monroe to install a direct fit bolt in y-pipe cat. Once I got the Blazer back from them I could hear this noise and it's been driving me crazy ever since. Looking around underneath the vehicle, the exhaust is installed correctly, does not rub anywhere, so I'm not sure what they could've done to cause this noise that sounds like it's coming from deep inside the dash, looks just fine.

I'll attach some youtube links, however I can only hear the sound in these recordings with headphones on. I cannot hear it on my laptop speakers, or TV speakers. So keep that in mind.


The video starts with the blazer in Reverse which you cannot hear the noise, and then I proceed to shift between park, reverse, and drive several times and you can hear the engine noise change. The noise that I'm talking about you can hear best at about the 30 second mark.

Can anyone tell me what it could possibly be?

 

hard to hear it as im at work and its pretty noisy. sounds like a whine, could it be a alternator back feeding? try taking your engine belt off and see if it goes away. also i remember running into s10s where the accelerator cable would end up resting on the pcv causing a weird side in the cab, that should be easy to check

 

Yes, the alternator does have a whine. I've replaced it with another whiny alternator, but the noise unfortunately follows. At least I think it's the alternator, I'm getting a stethoscope to make sure and further diagnose.

I've narrowed it down to either something in the HVAC system (I need to replace the broken floor heat flapper anyway) This will require removing the dash to get at, or doing the shortcut and cutting in through the floor.

Or, something noisy in the distributor, but I think this is not likely.

OR, it's the new cat that I had installed (new direct fit Y-pipe from Walker exhaust). The cat is very noisy when I went under the car with the engine idling. It reminds me a bit of my '86 Pontiac that had a clogged cat, so I gutted it as a quick fix, and it sounded very hollow, sorta growly.

I'm leaning towards the new cat being the culprit of the noise, which sucks, because that Y-pipe was relatively expensive, and it cleared my codes, so it's working well. I don't want to replace the cat just for noise unfortunately. Maybe Walker will warranty it, I can try but doubtful, since it was bought off ebay.

 

Nothing in the HVAC system is making this noise, at least that I can tell.

The whine is still there. I just rebuilt the alternator with all new internals and the sound follows.

I'm at a loss, no idea what it could be. The stethoscope pretty much pinpoints the whine coming from the alternator, but swapping/rebuilding the alternator doesn't solve it.