Engine Whistle

[Classics Fan]

1971 Jimmy has original 350 with 120,000 miles, rebuilt at 85,000 miles. A few days ago during warm outside temps it had a loud whistle coming from under the hood at start-up and idle (around 800 RPM). It went away with some throttle but returned when the engine slowed to near idle speed. It didn’t change pitch with acceleration, just faded away. This happened a few times. No unusual sounds at higher engine speeds or cruise. When the weather cooled the whistle went away too. I wasn’t able to get under the hood to check further. Any ideas what could cause this?

[SeventyOne]

I'd say it's almost definitely a vacuum leak.

[Classics Fan]

Originally Posted by SeventyOne

I'd say it's almost definitely a vacuum leak.

Pretty confident it’s not hoses or plugs so I’m checking intake and carb base torques.

[SeventyOne]

Originally Posted by Classics Fan

Pretty confident it’s not hoses or plugs so I’m checking intake and carb base torques.

check the carb to intake gasket, but might as well look over all the hoses/plugs. be sure not to over-tighten the intake or carb bolts.

if you can re-create the sound at idle open the hook and get a closer look.

[Classics Fan]

Originally Posted by SeventyOne

check the carb to intake gasket, but might as well look over all the hoses/plugs. be sure not to over-tighten the intake or carb bolts.

if you can re-create the sound at idle open the hook and get a closer look.

All hoses and plugs seem tight and secure. The engine runs well without miss or hesitation. I checked torque on the intake and was surprised to find 3 very loose, barely hand tight. I torqued all to spec. I rebuilt the Qjet a few weeks ago but failed to follow up afterwards, after several heating/cooling cycles, to retorque the hold down bolts. All 4 were quite loose so they got retightened to spec.

Forecast for next week is warm temps so hopefully I can check then to see if the whistle has disappeared.

[hemi43]

clutch fan?

[Classics Fan]

Originally Posted by hemi43

clutch fan?

It was a definite whistle, as opposed to a squeak or squeal. So far so good, I haven’t heard it again since tightening the intake manifold and carb hold-down bolts. Temps are forecast to be in the high 80’s this week so that should be the real test.

[scott123]

When at idle spray some carb cleaner around the suspected vacuum leaks.

If the idle or tune of the engine changes that is the culprit.

And since it is 2023, and not 1973, do not let the engine come to temp and then spray the carb cleaner on the exhaust manifold/headers. A friendly disclaimer. LOL

S