Strange new noise

Not sure if this is the right section but here goes. My 78 KZ 1000 has developed a new noise. Every now and then, on first start up or after sitting and cooling completely, it will have a whistle that sounds like an induction noise or big vaccum leak. The noise only lasts for 3 to 4 seconds and sounds like a vaccum chamber filling. After the initial start up I can restart the bike with out the noise returning. I've checked the induction system for leaks from the air box to the carb mountings at the head with both carb cleaner and my ultrasonic leak tester and so far have nothing to show for my efforts.

The noise seem to start after resetting the points, timing and idle speed. It was idling high and after the timing and dwell adjust the bike idles lower and runs smoother than ever. The only other thing I've noticed is that it seems a little slugglish as the motor gets up above 4,000 rpm. (If it were a car I'd be checking for a plugged converter or muffler, but, I don't think I've ever heard of this happening on a bike. Although, this bike did set for several years and even with the mufflers not sounding blown out, I guess they could be a pile of rust inside.)

Has anyone out there had a similar problem or have any additional input or ideas as to what may be causing this noise.

Thanks in advance for your assistance.