I have an '86 Civic CRX (also 1.5 I think; or maybe it's a 2.0). After going through "carbeurator hell" and making the adjustments to idle normal, I noticed it doing something similar.

When it does finally idle without me having to give it any gas (takes anywhere from a few mins to ten), it does the same thing where it'll idle at approx. 900 for maybe 2 seconds, then it drops (idle goes down fast and acts like its going to stall) to about 500-700, it then instantly makes a "submarine ping" noise jumping up to maybe 1100 and back down to 900

Everytime it drops, it does that sub noise and it goes through a never ending loop (does those 3 rpm drops & rises).

My other buddy took a can of brake cleaner (not sure if that's exactly what it was) and told me that when he sprays near the carb and its idle goes way up and holds for a few secs; that it'd probably mean that there's a vacuum leak somewhere - so he sprayed near all the hoses (30+ of them on my carb) and finally it did it when he sprayed near the bottom of the carb and asked if it had a carb gasket, (thin cardboard-looking thing that sits directly between carb & engine).

haven't gotten around to getting that piece since it doesn't really bother me, but I have been getting probably lately from staring up without having to use starting fluid or pouring gas into the carb; and it seems like the front driver side ??boot?? broke (sounds like breaks grinding) so I gotta get a new axle.