Can't identify this noise

[Bobbed72]

Have a carbed 5.3 ls motor that makes a seriously screech when revved aggressively. Doesn't do it with gradual acceleration.

https://youtu.be/ZSNEbdQ7Bj8

Since it's carbed I figure I will have more luck in this subforum then the LSX forum.

The recording doesn't convey how loud it is. Had someone tell me it was normal airflow through the carb but after he heard it doesn't know what it is.

[geezer#99]

Quite a lag when you stroke the throttle!
What’s your idle rpm?
What’s your timing curve
like?
What carb?

[mattfranklin]

Sounded like kind of a whistle sound, like a bad vacuum leak, except vacuum leaks are loudest at closed throttle. I had that when a chunk of the carb-to-manifold gasket was missing on a Ford 360/390. So it's different. But sometimes things just whistle a bit.

Does it change with your air filter off? Can you test it by capping off all other air fittings into the manifold one at a time and listening each time? I'm thinking of something funky with a brake vacuum or PCV or heater damper or other vacuum assisted device.
At least if you test those you can check them off the list of possible things wrong.

[Wrenchbender Ret]

Sounds like a slipping fan belt.

[Bobbed72]

Originally Posted by geezer#99

Quite a lag when you stroke the throttle!
What’s your idle rpm?
What’s your timing curve
like?
What carb?

Fixed the lag since taking the video. Idle screws where incorrect.

Idle RPM no load it around 900. Drops to ~600 in gear.

Timing curve is around 13 at idle but varies from 24 to 34 depending on manifold pressure. Cruising at 1800 it'll be 33-34 then drop to 28 when pushing 4000.

Running a quick fuel brawler Vac Secondaries.

Originally Posted by mattfranklin

Sounded like kind of a whistle sound, like a bad vacuum leak, except vacuum leaks are loudest at closed throttle. I had that when a chunk of the carb-to-manifold gasket was missing on a Ford 360/390. So it's different. But sometimes things just whistle a bit.

Does it change with your air filter off? Can you test it by capping off all other air fittings into the manifold one at a time and listening each time? I'm thinking of something funky with a brake vacuum or PCV or heater damper or other vacuum assisted device.
At least if you test those you can check them off the list of possible things wrong.

I tried spraying around the manifold and carb for a leak. I may try capping vacuum lines today. Map sensor, brake booster and Transmission are the only connections to the manifold. Same result air filter on or off.

[mattfranklin]

Originally Posted by Wrenchbender Ret

Sounds like a slipping fan belt.

That's a good one. So tighten the belts and see if it changes.

[geezer#99]

Interesting timing curve!
You are checking your idle timing with thevac advance plugged off?
At 900 your mechanical timing is kicking in. So your actual initial is likely a lot lower.
Your 900 idle and 650 drop in gear tells me your primary is too far open. You’ve likely uncovered the transition slot.
That’ll make it whistle when you crack it open fast.

[Bobbed72]

Originally Posted by Wrenchbender Ret

Sounds like a slipping fan belt.

Originally Posted by mattfranklin

That's a good one. So tighten the belts and see if it changes.

Considered that as well. Fairly new belt. The whole accessory setup is custom. Had to find a belt that fit. Previously had a very tight one on that was causing a bad noise. Went the next size larger that the parts store had and it sounded better.

Looking at again and the belt seems to have stretched. Tensioner is barely pulling on it now and the back of the belt looks like has spun. Looking for a belt in between the two sizes.

[Bobbed72]

Originally Posted by geezer#99

Interesting timing curve!
You are checking your idle timing with thevac advance plugged off?
At 900 your mechanical timing is kicking in. So your actual initial is likely a lot lower.
Your 900 idle and 650 drop in gear tells me your primary is too far open. You’ve likely uncovered the transition slot.
That’ll make it whistle when you crack it open fast.

Don't have a distributor. Timing is handled by an MSD box for LS motors.

I'll look into the primary. But I'm leaning towards the belt being the source since it is loose and a sudden acceleration would cause a bit of slippage.

About to test it here.

[Bobbed72]

Belt. Didn't sound like a belt but it's the belt.

Thanks, Wrenchbender.

[YoungPup1977]

When experiencing any noise the first thing to do is start eliminating things. Like for an example, removing belts......torque converter bolts, disconnect vac line to brake booster., etc. etc ..... plus it helps to know what was done to the vehicle lately. Was it a new belt ? Lol!!!!
Posted via Mobile Device

[Bobbed72]

Originally Posted by YoungPup1977

When experiencing any noise the first thing to do is start eliminating things. Like for an example, removing belts......torque converter bolts, disconnect vac line to brake booster., etc. etc ..... plus it helps to know what was done to the vehicle lately. Was it a new belt ? Lol!!!!
Posted via Mobile Device

Yeah it was a new belt. Originally I used a 77in belt. It was pretty tight and started to cause noise like it was too tight for the pulley bearings. So I went to a 78 inch belt cause that's what the local parts store had (I was out of town) and it worked fine. The noise doesn't happen during normal driving. Only noticed it when tuning the carb. Think the RPM's had to increase pretty rapidly to get the belt to slip. I have heard slipping belts before. Like when first starting a cold motor. But this was new to me. Sounded like a whistle.

[Ironangel]

Glad you found it...I was about to have you climb underneath and check that flexplate...