Noise, can't find souece

I just bought a 1998 Dodge Durango 318 auto. trans 71,200 miles, from a private party. I took it for a test drive a few days ago for about 5 miles, and I left it ideling in their driveway when I got back for about 5 minutes. The engine was at operating temperature by now. Never heard this noise. I just brought it home, about 12 miles and when I got in my driveway, I could hear a dull thumping noise at about 800 RPM. If I sped the engine up about 200 rpm, I could still hear the noise barely, and then it disappeared. I can't hear anything under the hood, but not entirely sure. I hear the noise the best when sitting inside with the windows up. I took the Durango right over to my mechanic, this is at night near closing time. He heard the noise, and thought it might be a baffle or something in the muffler. Here is the crazy part. If I now shut the engine off (now remember it is already hot)and then start the engine, there is no noise. BUT the noise will return in about 1 - 2 minutes. It is almost like a switch or something is turning this noise on and off, because when I am waiting that minute or two, the noise doesn't gradually come back. When it returns it is instant. I have checked the exhaust pipes, muffler, catalytic converter etc. It sounds like the noise MIGHT be coming from the cat. converter, but I can't say for sure. This is all I can relate for now. Any opinions or suggestions ? Thanks, Mike

The catalyst in the catalytic converter is likely loose. For months I could swear there was someone under my truck making the noise, and I eventually found out it was the cat failing. I bet that's what is happening here.

Hi BleedDodge, Thanks for the reply. I did think the noise was from the cat, especially since it is right under the seat where I hear the noise the best. But I would think the catalyst would make the noise constantly after the vehicle was hot. And like I said in the post, if I turn the vehicle off for a second or two and then restart, the noise is gone for about a minute and then returns. Did your problem act like this ? Thanks, Mike

It came and went. It seemed to occur more when I was turning corners and it would shift to one side inside the cat, things like that. If I hit a bump you could hear it loud too. Most of the time, when you suspect the cat is to blame, you should just have it replaced for good measure. These things will leave you stranded. Your warranty will probably cover it for free.

I found the source of the noise. There is a small unit mounted on a standoff bracket that is mounted on the firewall rear of the engine. There are a few hoses that go to it, and also a wire. If I feel this unit I can feel a pulsing or tapping inside. And if I remove the wire that goes to it, the noise stops. There is a small green screw off cap about half inch on top. There is also a tag that reads "EVAP. SERVICE PORT, 1 PSI MAXIMUM REGULATED PRESSURE ONLY" What exactly is this thing, and does this unit need to be replaced, or is it just louder than normal ? Also, is this a Dodge item or can I find it at NAPA or Parts America ? Thanks for your suggestions and thoughts, BleedDodge !

Sounds like you found the evaporative canister.

If there's something wrong with that thing, it should be setting off the check engine light. Has it come on at all?

No, the check engine light has not come on. This unit I am talking about is pretty small, don't think it is a canister. I could almost cover the whole thing in my one hand. I heard the term "Purge solenoid " somewhere. Can that be it.

I had the same thing happen, and the mechanic said if I replaced it with A new one, with in 6 months I would hear it again. It still works fine.