When I'm sitting in traffic, in neutral or park, or just after I've parked up, with the engine running, I hear a single, solitary, loud clang from under the car.

It's not the aircon as it's turned off.

It does it a couple of times a minute.

Any ideas?

hey,, that's your wife under there, after you ran her over !!:halo: :halo: :bonkers:

If it's a higher pitch rattle I would suspect the catalytic converter heat shield. They are spot welded on and break loose after becoming brittle with all the expansion and contraction. That would be the first thing I'd check. If that is the culprit you can have it welded back on or you can remove it all together, just don't park in tall grass or weeds after a long drive!

Hehe, no not the Mrs, although maybe I should retro fit a sunroof to the floorpan just to be sure?? <grin>

It's not a rattle.
It's as though someone got a hammer and gently hit a steel pipe.
It's not continuous, just one loud clang then maybe another 30 seconds (or whatever) later and so on.

I thought it could be the aircon pump cycling but I turned that off and it still does it.

Then I thought it could be the exhaust contracting as it cools but it's not like that tick, it's a real loud clang.

Hehe, no not the Mrs, although maybe I should retro fit a sunroof to the floorpan just to be sure?? <grin>

It's not a rattle.
It's as though someone got a hammer and gently hit a steel pipe.
It's not continuous, just one loud clang then maybe another 30 seconds (or whatever) later and so on.

I thought it could be the aircon pump cycling but I turned that off and it still does it.

Then I thought it could be the exhaust contracting as it cools but it's not like that tick, it's a real loud clang.

It could be the catalytic converter. If it's shattered internally, the pieces can shift around and make loud noises like that. Outside of that, maybe a heat shield is not held up properly as Kev suggested or one of the rubber exhaust mounts is missing. There's really nothing else moving down there, and the exhaust isn't bolted to the body, it just hangs there on rubber mounts so it doesn't break when the car flexes. Since that's about the only thing that can move, it pretty much has to be something exhaust-related if it's coming from underneath the passenger compartment. Do you have the facilities to crawl under there and look?

Yeah me and a buddy made some ramps out of wood to drive up. They're like 6 foot long and raise the car 12 inches.

It may not be underneath the car (can be difficult to tell where sounds are coming from). It could be exhaust related though, I'll check.

Had a failing A/C clutch on a 97 ETC that did that. Didn't matter if the A/C was on or not. It's located low, bottom front of engine. I had trouble locating the sound from on top. I had to actually crawl part way underneath with the engine idling. Scarey.

Interesting.
I'll check that out.