"popping" in 88 Subaru GL

I have an 88 Subaru GL that is making a somewhat loud"popping" noise when pressure is released from the gas pedal. The sound is intermittant, but becoming more frequent, and stops completely with no gas pressure or full throttle. The oil pressure remains constant at 40 so I don’t think it is any type of engine bearing. I took it to the shop and of course it did not make the sound when they drove it.Sounds like it is under the hood, maybe to the passenger side. No clicking or noise when turning.

I should add that I am getting a code 24 (coolant temperature sensor or circuit, or Idle-up solenoid or circuit (1984 to 1988 carbureted))error that I originally thought was blinking a code 13 (crank angle sensor). The noise seems to rattle off at the same frequency no matter the speed or gear. The car is not overheating and it is not a boiling radiator sound.

How has the fuel economy been? If the coolant temp sensor is not working correctly, perhaps the engine is not getting the correct mixture, causing some minor popping back through the air intake.

I drive 80 miles a day, mostly highway at approx 32 mpg. I haven’t noticed a drop in the mileage

Well, if your engine light is telling you to replace that sensor, I’d do that and go from there. Try and figure out if the popping sound is coming through the air intake. There could be something out of adjustment. I’ve heard that sound before coming back through air flow meters near the air intake on older (primitive) fuel injected vehicles.