Crackling/grinding noise

[jojo-munson]

Hey all, recently my 72 c20 350 has been making an occasional crackling/grinding noise, and the Google can't find an answer. The noise lasts 1-2 seconds then stops. It's not consistent and I can't seem to reproduce it on command. Originally I noticed it while driving in first at idle, and thought it was coming from under the cab. But yesterday after replacing a cracked exhaust manifold and while letting the truck idle in park, it made the noise. Except this time it sounded almost like it was coming from the engine. I'm scratching my head trying to figure out what it could be.

Some things I've done already. Checked the power steering pulley and alternator for wiggle. The power steering fluid is also at the correct level. The u-joints look good too. I fixed a broken exhaust hanger so the pipes are pretty sturdy. I have an oil leak and super small transmission leak (Cherry red on the stick though which is great), but those have been there since I got the truck and this is a new noise.

Do y'all have an suggestions for things I should check? I know sounds are real hard to describe without actually hearing them, but I can't seem to catch it on video. Thanks in advance.

[jojo-munson]

So after some tinkering I realized that I was running the engine rich. Leaned out the idle mixture screws and amazing how that potent exhaust smell went away haha. I'm slowly building a repertoire of smells to recognize at least.

But what I did find was that on the passenger side, the front bumper is actually hitting the grille. And that my bumper is no where near level. I've wedged a little piece of cardboard in there for now, but how would I fix this? The bumper was never super level to begin with, but looking at older pictures there definitely used to be more space between the bumper and grille. The way it's bolted to the frame makes it look like I wouldn't be able to wiggle it down either.

[Steeveedee]

I can see the gap between the grille and the bumper gradually get smaller as I look across from driver's side to passenger's side. You should be able to loosen the bumper brackets at the frame and pull the bumper forward. Take a look at the brackets on each side and see if the ones on the passenger side are bent. It could be that someone hit something before you got it, and only replaced the bumper without straightening or replacing the brackets.

[jojo-munson]

I'll give that a shot, thanks!

[71threequarterton]

Originally Posted by jojo-munson

But what I did find was that on the passenger side, the front bumper is actually hitting the grille. And that my bumper is no where near level.

Mine has basically the exact same problem. Going to check to see if it's a bent bracket.

[my67c20]

Damn mysterious noises. Ran into a similar situation years ago that sometimes when I hit a bump i would get a loud thud. Parked the truck checked all the suspension parts, welds, etc. Found a few sketchy things repaired them, but the noise still existed. So I mentally tried to keep track of what road and what I was doing on the road that recreated the thud on a regular basis. Found the secret turn that I could duplicate the noise, but I still could not find the issue. Took everything out of the bed and the cab that could move, noise was still there. Well one day my wife was following me home and she tells me that when I made the turn it looked like the bed was going to fall off. Low and behold, the thud I kept hearing was the left front corner of the bed, slamming into the cab every time i turned left and hit any kind of bump. Long story short, check every nut and bolt.

[Barnfind46]

Keep checking and eliminating the little noises and rattles. It seems hopeless at times but eventually you will find the source of them. I had a loud metal to metal banging when I drove on rough roads. I looked at the suspension and exhaust. Come to find out the sway bar frame mounts had no rubber between the sway bar and frame but looked ok from the other side. That one took me a while to find!

[thedudeabides86]

Originally Posted by jojo-munson

So after some tinkering I realized that I was running the engine rich. Leaned out the idle mixture screws and amazing how that potent exhaust smell went away haha. I'm slowly building a repertoire of smells to recognize at least.

But what I did find was that on the passenger side, the front bumper is actually hitting the grille. And that my bumper is no where near level. I've wedged a little piece of cardboard in there for now, but how would I fix this? The bumper was never super level to begin with, but looking at older pictures there definitely used to be more space between the bumper and grille. The way it's bolted to the frame makes it look like I wouldn't be able to wiggle it down either.

Attachment 1829674

Attachment 1829675

Speaking of running rich, my 72 C20 is doing the same thing. A guy at the shop mentioned it was running rich and that he could smell it, so I've been meaning to adjust the idle mixture screws. Without me being a pain in the ass, is there an easy way for you to explain the way you adjusted yours? I have a Holley 4 barrel and I know my way around most of it. I'm just still unsure of what exactly to adjust so as to not screw up what's working, for the most part. I hope you solve the case of the mystery noise.

[weq92f]

Chasing down chassis noises can be a challenge. Keep at it and you'll find it. Grab a ratchet, a 3/8, 1/2, 9/16, 5/8 and 11/16 for starters and climb under there and just start checking every bolt and nut you can get on.

My last hunt for a clunk found a passenger core support mounting bolt loose as a goose!

More recently I was hunting a noise from the front of the engine that came and went with warm up and cool down. Turned out to be the accessory belt!

Good luck!

-klb

[jojo-munson]

Phew that's quite the list of things and places to check, thanks folks! The more I work on this truck, the better I'm getting at guessing socket sizes . Hopefully this weekend I'll have the time to get out there and hunt for the noise.

@thedudeabides86, I followed the steps in Edelbrock's tuning guide, I have an AVSII. Not sure if the Holley will be different. I've pasted the steps I used below. Well, mostly used. I did it roughly by ear for now, but once I have the time I'll properly do it.

Based off the steps below, using the idle mixture screws I found max RPM (honestly leaning it out at first upped the RPMs which was a bit odd), then leaned it until the rpms dropped a bit, and lastly I upped my idle speed back up. Still not the happiest sounding engine at idle, but she revs nice. (On my carb, turning the screws clockwise leans the carb)

1. Fully warm engine and ensure choke is fully open.
2. Air cleaner in place.
3. Set desired speed with the idle speed screw.
4. Adjust the IMS on ONE side to get the maximum possible RPM. Do not go rich beyond the
maximum speed point.
5. If the above changed the idle speed more than 40 RPM, then readjust the speed.
6. Adjust the side OPPOSITE of that in Step 4 to get maximum RPM.
7. Reset the speed.
8. Carefully turn each IMS to again get the maximum idle RPM.
9. Go leaner just enough to get a 20 RPM drop in speed.
10.Reset the speed to the desired RPM.
11.This is a Lean-Best Idle Set. Setting richer than this will not improve idle quality or
performance, but could tend to foul plugs.

https://advertising.edelbrock.com/Li...Guide_0348.pdf Page 4

[jojo-munson]

Terrifying how long it took me to finally get under the bumper and mess around, but such is life.

Looking at the bumper mounts, I did not see how I could shift the bumper into the correct position. I still loosened every bolt and tried, but no luck. Issue is that the bottom of the bumper is contacting the bottom of the frame, so there's just no room for the one side to move up.

The struts look okay, at least to my eyeball, I don't think one is bent.

Bumper/frame contact on driver side


driver side strut


passenger side strut


I think I'm going to get a thick washer or 2 and space the bumper on the passenger side forward a bit so that it doesn't hit the grille anymore. Until it gets dry out, then I'll try and take the bumper off and see what I can do. I'm wondering if the bumper is just warped a bit.

[sick472]

I hope I am wrong, but the passenger side section of the bumper appears to be twisted (clockwise as viewed from the passenger side looking at the side of the truck) so that it has moved the upper edge upward. Look for damage on the lower section of the front of the passenger side bumper that could have caused the twist.

If the twist is just an allusion in the photo, I would have to remove the outer angled gusset brackets and see if I can get the bumper to set correctly by just using the frame connections. This would prove that the frame is not bent. Then add the angled gussets back.

[71gmcC15]

I'll be driven down the highway and it will sound like some one chuked a rock at the back of the cab. Happens all the time. No idea why. Happens randomly. As for the bumper it could be bent too.

[Curtb1971]

Just a guess here, but maybe take a look at your radiator core support bushings? From your pictures it looks like the passenger side bushing may be missing some rubber in between the frame mount and the core support.