[Bobcat]

Stuart .... air bleeds shouldn`t make any noise . As Patrickt suggested , check for air leaks around the base . You might try some WD 40 as it will cause the engine to speed up if you spray where the air leak is . However , if the engine idles and runs extremely well as you indicated , you probably don`t have an air/vacuum leak . Dumb question ... you said you replaced the base gaskets for a 4 hole gasket .Is your intake also a four hole unit ? If so , then the 4 hole gasket will work , if it is an open plenum , then a four hole gasket can cause some problems as it isn`t supported in the center .
Sometimes one mixture screw can be off and that will cause that venturi to make a noise ... mine did . Try adjusting the idle mixture screw on that side and see if the whistle goes away ( make a note of where you started so you can have a baseline ) . I always make a note of where each mixture screw is on my carbs after they are adjusted ( how many turns out from closed ) .... gives me a baseline to go back to if I start changing things and get absent minded .

Bob