I mention the trailer because I was driving almost the whole way with the overdrive disabled, so the engine was running about 2700 rpm at highway speeds. And it was warm today, ambient temp was about 85 degrees F.
So when I get home, after getting off the e-way, I have about 3 miles to go, up a couple of hills. When I got to the house, I heard it again, and this time I got right out and opened the hood. Not coming from the top of the engine, so I got underneath. It sounded like it was coming from the right side of bottom of the engine. It could have been coming from the transmission, but it was difficult to tell...I was using a long wooden dowel against my ear, checking different locations, to isolate the noise. But after about 2 minutes of idling, it went away again!
My first guess is a possible leaking exhaust manifold, as (a) I live in the Rust Belt and my exhaust system is becoming more oxide than iron , (b) the exhaust has always had a resonance rattle at about 2000 rpm since new (the Y-pipe back had been replaced under warranty twice and it always came back) and (b) I am hoping like hell it's not inside the engine. I didn;'t get a chance to check the manifold before it stopped, but I did check the V-pipes under the pan and that was inconclusive. My guess is after running at street speeds for a few minutes (and towing the trailer), the exhaust was heating up more than it did at highway speeds (with less airflow to help cool it), and something in the exhaust was expanding enough to cause a leak. After it idled a little, it cooled back down and stopped leaking. I drove around without the trailer and it never came back. All fluid levels OK, oil pressure is there but of course without hooking up a gauge I don't know the exact pressure.
Any other suggestions?