I have had my 84 for just over a year now, and it's always had a slight sound that I've found annoying. I only noticed it at idle, but then I probably wouldn't notice it at an RPM or a speed much above that. I would guess it's what other members have been calling the Jetson car sound...sure seemed like that to me.
Well I've finally traced mine down to the YICS system. I guess that after exhausting other possibilities, it just became necessary to check. I tried spraying the system, but never noticed any change in sound or rpm. So I pulled off the YICS chamber(s) and plugged all of the ports/hoses on the engine...and the sound has gone away! I submerged the YICS chamber(s) and found a tiny, tiny leak at the seam on one of the chambers. It still seemed to hold vacuum pretty well, but under pressure there was 1 bubble escaping every couple of seconds.
Just thought I'd share what I'd found, since it seems a pretty common malady.
The Jetson car sound is gone...but now I can hear the clutch!