My Quadrajet developed a whistle this weekend. I haven't touched it in a while and there has been no change in performance. I can hear the sound with the air cleaner on. Has anyone ever encountered this?

 

vaccum leak?

 

Mine started to whistle increasingly (at idle) a year or two after I bought the car. It emanated from the base of the carb and turned out to be a failing carb - manifold gasket.

 

Is there any easy way to test the gasket?

 

with the vehicle running get a squirt bottle of anything really or some wd-40 and soak the base of the carburetor all around where the gasket is and at the manifold (if there is a spacer) and all gaskets inbetween and if the whistle goes away then you found your problem

 

instead of spraying oil all over your manifold substitue brake clean or starting fluid and spray around the base of the carb with it running.. if the idle changes you found the problem.

 

Would a leak at the manifold/carb gasket cause a lean condition?

 

Are you saying you have a lean error? Under what circumstances?
A leak might cause leaning at idle but not necessarily under load.
The more you open the throttle the less a leak will influence the mixture.
This is due to two factors; there's less vacuum then to pull false air in, and there's so much more air coming in through the throttle than at idle.
If you have a couple of feet of hose you can put it to your ear and use the other end to pinpoint sounds.
You might of course have additional problems also, like an exhaust leak or a bad O2 sensor for instance.