440 Vacuum Leak, where to start?

I have finally got my 1970 Duster with 440 on the road. One niggling problem that I am having seems to be coming from a vacuum leak. The motor was running an AFB Competition 750cfm carby, when idling it would run a little rough with a big flat spot off the mark, then under load a loud whistling noise would come in and remain until the motor is turned off. When restarting the whistling would be gone until under load again. I replaced the carby with a brand new Holley 750, motor now starts a lot easier, idles fantastic, no flat spot...but...it still whistles. Not quite as bad as before but enough to be annoying. I suspect the leak may be coming from the 1" Phenolic riser that is under the carby as I did not remove this when I installed the new carby. This will be quick and easy to check, but I'm not sure where to check next if this is not the source. I guess what is throwing me is that the noise doesn't come in until your in gear driving. Could the leak be associated with the 727 ?