Thread: Rumble at idle with occasional popping sound

So, first post on here, sorry for it being a problem. I usually try to help a few people on these forums a few times before I go asking for it, but this one has me stumped. I've got a 98 GTP with 154k on it, and it has developed a funny rumble at idle. It's not there when the engine is cold, but as soon as it hits about 160 degrees it starts to show up. I've tried recording it, but it's too quiet for my phone to pick up, and the really weird part is that you can't hear it under the hood, only inside the car. The best way to describe it would be that it sounds like a pot with a lid on it when the water starts to boil, just kind of a rythmic rumble. The next development has been that there is the occasional popping sound that comes with it, almost sounds like microwave popcorn. It'll do it maybe once or twice, stop for a few seconds, do it again, not do it for a minute or so, do it again. Totally random. The rumble goes away as soon as I touch the throttle, and the popping goes away after about 1500 rpm. No drivability issues, no idle issues other than the noise, and no starting or power issues. I got one check engine light for a camshaft position sensor, replaced it, and no change other than gaining 2 mpgs. Totally stock motor with the SLP air box. Love this car, and I'm just trying to keep it in as good of condition as I can. Any ideas? Any help would be great. Thanks in advance.

Sounds kinda like a misfire at idle.

Wouldn't that throw a code at least, or cause a noticable drop in rpm at idle?

The occasional idle misfire is fairly normal. It wouldn't drop the rpm is it's not to bad and wouldn't throw a code unless it was pretty bad.

With it happening only after the car is warmed up, would you think more coil packs or plugs? I seem to remember these coil packs being known for failing once they warm up when I was managing for Autozone. I'll look up the resistance on them and fire up the old multimeter I guess.

stock coils are the best you can get. Try new plugs and see if it helps.

I was just saying the 3800 coils in general, not just the Autozone ones. Sounds like new plugs are on the agenda for tomorrow. Thanks!

So, changed out the plugs and wires tonight...no luck. No change in the sound, although I don't think I heard any popping. The rumble didn't change at all though. Any suggestions? I think tomorrow i'm going to try running it with no belts to see if it goes away. I know my power steering pump is on it's last leg.

Try running some seafoam through it while your at it. It might not need it but it surely can't hurt.

Didn't get a chance to take the belts off today, but I put half a can of seafoam in the crankcase and in about 15 miles you.could barely hear the rumble anymore. Tomorrow i'll run the other half through the vacuum syslem and I think we'll call it a success.

So, update on this. I don't know if it was the weather, or the engine not quite warmed up or what, but it hasn't quit. I took the belts off and ran it, but it still sounds the same. I'm totally stumped on this. No drivability issues, no drop in rpms, just this kind of popping sound. I had a Volvo that made a sound kind of like this, but only when turning at low speeds and that turned out to be a motor mount. Does this seem likely? Other than that, it has to be internal engine, but it runs perfectly and doesn't seem loud enough or constant enough for that.