Nasty smell in Subaru Outback

The car is a 2006 Subaru Outback with 82,000 miles. At low speeds we sometimes get a strong smell like natural gas in the cabin. Took it to the dealer twice, but it behaved — no smell.

The car has mostly been driven short distances around town, with occasional longer trips. More recently, in the past 18 months, it’s gone to New York City 7 times, round trip distance about 1500 miles.

Three weeks ago it had its head gaskets replaced. Also fluid replaced in the front and rear differentials.

After that we started noticing the smell. It’s not there all the time; when we first start the car there’s no smell, then after a bit there’s a burst of odor that persists for a while, and comes and goes during low speed driving. Our last trip to New York was after this started; no smell at highway speeds. But after the trip the smell was still there at low speeds. Just not when the service folks are looking at it.

I just got it back from the dealer after its second visit. They kept it all day, and had four people drive it around town. Nobody smelled anything, except the service manager who mentioned that once he got a slight whiff of natural gas, but he decided that was from outside, since there’s nothing in the car that would smell like that. But that’s exactly what I smelled. (and, no, it’s not hydrogen sulfide; I know what that smells like).

So I’m appealing to the wisdom of the Internet: does anybody have any idea what might be causing this?