Whistling holley 1920
Often fixable with some carburetor cleaner spray; with the engine running at a fast idle, put the end of the straw on the larger of the two air bleeds facing the sky, visible by looking down into the carb throat, and spray. Then repeat with the smaller air bleed. Once that's done, adjust the ignition timing—yes, it —and the idle speed properly (not by guess-and-golly). On carbs like this, with the throttle plate hole, the throttle plate is meant to be fully or very-almost-fully closed at curb idle. You set the mixture as described here, (the idle speed instructions there are for carbs without throttle plate holes), and you'll also find carb operation and repair manuals and links to training movies and carb repair/modification threads at that same link. If your '68 has the distributor vacuum control valve, that has to be adjusted correctly as well, or you'll get weird problems which can include difficulty getting the correct idle speed and (yep!) whistling at the carb.
Often fixable with some carburetor cleaner spray; with the engine running at a fast idle, put the end of the straw on the larger of the two air bleeds facing the sky, visible by looking down into the carb throat, and spray. Then repeat with the smaller air bleed. Once that's done, adjust the ignition timing—yes, it —and the idle speed properly (not by guess-and-golly). On carbs like this, with the throttle plate hole, the throttle plate is meant to be fully or very-almost-fully closed at curb idle. You set the mixture as described here, (the idle speed instructions there are for carbs without throttle plate holes), and you'll also find carb operation and repair manuals and links to training movies and carb repair/modification threads at that same link. If your '68 has the distributor vacuum control valve, that has to be adjusted correctly as well, or you'll get weird problems which can include difficulty getting the correct idle speed and (yep!) whistling at the carb.
Yes, it is. In the '68-'69 timeframe, all the US carb makers were starting to transition away from the old method of cracking the throttle plate open a bit to provide idle airflow; instead they went to consistent, fixed idle airflow by closing the throttle plate completely at idle and providing a calibrated hole in the throttle plate, or no hole in the throttle plate but an adjustable idle air bypass pathway in the carb body as shown here:
