69MustangCoupe393cid

Not a huge problem because the car runs and drives fine, but sometimes it idles weird. I think it might be a vaccume leak or something. It only happens when the engine is loaded, and about 10-20% throttle (I can't hear it just revving in my driveway), anything before or after that % throttle it goes away. It's a very loud hissing sound that sounds like a bad vaccume leak, does anyone have any clue as to what this might be? I tried spraying carb cleaner around at idle to find a leak, but I can't find anything.