02 Superduty Crew Cab v10, 62,000 miles. Today I towed my trailer loaded
with a Kawasaki Mule and a John Deere lawn tractor today from my cabin in
North Michigan to my home in Southeast Michigan, about a 3 hour drive. When
I stopped for lunch at about the halfway point, I could hear a fast
'tck-tck-tck-tck" that varied with engine speed. By the time I got out and
opened the hood, the noise was gone. OK, thought it might be a sticky
injector, didn't seem to harm the engine, and it did go away. Never heard it
before or after at street or highway speeds. No dash lights came on at any
time.

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?