Strange, LOUD noise after engine is turned off

[gmc396]

I have a '68 GMC 396 that I've owned for 7 years. It's a summer ride for me as I live in WI. I got it out last week and drive to get my mail daily. It's about a 5 mile RT.

When I pull into my garage, I typically turn off the ignition and let the truck roll into place. I do this to keep the exhaust smell out of the garage (the exhaust pipes exit just behind the rear wheels). Yesterday, when I turned the truck off, the noise began. It sounded like an electric motor building up in RPMs, but also very mechanical and loud. Never heard a noise like this before. When I came in the house, my wife said she heard it and thought I had my chain saw out.

I thought maybe the engine was dieseling (rarely does that) so I put on the brake and let out the clutch but the engine was stopped. But the noise continued. I was concerned of a possible fire so started the truck and backed it out of the garage.

I turned off the engine and opened the hood. The noise continued, very loud. I could see nothing. Finally it wound down like a big electric motor and stopped.

I couldn't see anything wrong. After letting it sit, I started it and put it back in the garage. No noise.

Any ideas of what that noise was???

Thanks for your help!

[Wootdog]

was the heater fan on by chance?

[Willys_MB]

Maybe your starter is running after you turn the truck off? That's the only electric motor I can think of other than the heater, but the starter is pretty loud. Does it do it if you stall the engine, too?

[hamjet]

Do you have electric cooling fans that run and possibly have a leaf or debris hitting the blades?

[Dragcamaro]

Greetings, Fellow badger!


If it does it again, are you able to disconnect the Negative battery cable to isolate if it was an Electric issue vs mechanical?

I'm curious as well on the fan, possibly unplug the heater fan while it's doing it (or the fuse for that circuit)? Could do same for starting circuit (remove the fuse) to rule that out as well? Possibly a bearing going bad in fan motor, or a mouse maybe made a home in there?

[gmc396]

Thanks for your comments and thoughts. The truck is stock so no electric cooling fans. I did stall the truck yesterday but the noise continued.

I looked in the fan shrouding for some pesky mice but everything is clear. I just turned on the ignition and ran the heater fan thru the 3 speeds but sounds normal.

Started the truck twice and normal. I'm going to drive to the PO now and see if it does it again

[toolboxchev]

You have an electric fuel pump?

[Astro-Balls]

Would a broken clutch fan keep spinning and make noise?

[Boog]

Next time it does it raise the hood and go over to the right side and bump the starter with something hard. The starter may not be fully disengaged after the engine starts, still spinning.

[Mike C]

Usually it is not that loud, but a harmonic balancer that has failed and the outer ring spins on the hub for a few seconds after the engine stops can make a racket.

[truckster]

Maybe post a video clip...

[gmc396]

Thanks again for the comments. The harmonic balancer is an interesting possibility as I imagine that could be noisy when spinning. I started the truck and looked at the bottom pulleys by the harmonic balancer and all looked good with not wobble. I imagine once the rubber part of the balancer fails the pulleys would no longer run.Plus I can't see that continue to spin for what was a good 30 seconds.

I'm wondering if there was some type of short in the starter circuit that did not engage the starter gear but fed power to the starter motor.

I drive the truck daily so will keep my phone/camera handy so I'm ready next time it happens.

[HO455]

Got any family, relatives, friends, or neighbors who love a good practical joke?

[Ol Blue K20]

Power brake booster with a pin hole in the diaphragm might do it

[gmc396]

Just for fun, posted a couple of pics

[Steeveedee]

I like it! Are the crossed flags factory trim? They look cool on there, in any event.

[gmc396]

No, the cross flags for the 396 were added; couldn't resist. Right below the GMC emblem on the front fender it says V-8 and C2500.

[vitamin_c95]

Auto or standard transmission? Just trying to think of other possibilities not mentioned.

[mikeshome]

he mentioned clutch somewhere in his post,

[mrtoni]

Posted via Mobile Device

[JT_Trucks87]

Very cool truck! interested to see what the noise ends up being.

[vitamin_c95]

Wondering if the throw out bearing is going bad? Far fetch just thinking of other things to look for

[gmc396]

It's a 4 speed "granny"

[Mike C]

Originally Posted by gmc396

The harmonic balancer is an interesting possibility as I imagine that could be noisy when spinning. I started the truck and looked at the bottom pulleys by the harmonic balancer and all looked good with not wobble. I imagine once the rubber part of the balancer fails the pulleys would no longer run.Plus I can't see that continue to spin for what was a good 30 seconds.

Nope, pulleys run just fine as they are attached to the hub. It is the outer ring that continues to spin when turning the engine off. It can move forward and backwards rubbing/grinding on the back of the pulleys or the timing cover.

[gmc396]

Just so I understand, the outer ring of the harmonic balancer is the part with the timing mark on it. If that has broken loose, should I be able to turn it? I used a long wooden dowel rod and tried to move it with no luck. Could it have temporarily broken loose; that doesn't make sense to me. I've driven the truck a half dozen times since the noise and all has been normal.

Thanks for you help.