I have a 96 Civic with 192k miles.

The engine makes an intermittent, random popping noise until it's warmed up. The noise increases in frequency and amplitude as throttle and rpm increase. Once warm, the noise goes away completely. I can prevent it when cold by going really easy (accelerating at 10" hg vacuum or higher), or by warming it up until it idles down to about 1000 rpm before starting out.

It started about 5yrs ago but only on cold winter mornings. Now it happens even on summer mornings.

It used to only happen under load in higher gears, now I can even get it to do it by pulsing the throttle under the hood by hand which finally helped me figure out a little more about the problem. I'll get one or two pops out of it that way before it gets too warm and it goes away. I'll say it sounds like it's coming from the intake side of the #4 cylinder / distributor area. I don't hear or feel anything from the injector.

Over the years I've replaced the exhaust manifold, tuned it up (wires, plugs, cap, rotor, timing check), adjusted the valve clearance and none of those had any effect.

Average oil change interval over last 120k miles is 3100 miles. PCV valve is new. Air filter good. No check engine light. NOT CV related (happens when stationary and not a rhythmic noise like a clicking CV).

Any ideas?