Casey Wilson wrote :
Something was rattling and thumping hard enough and loud enough
to
be heard over the muffling of my passive headset.

I figured it out when I read this sentence. You're not the first
and you certainly won't be the last. Just be glad that it was only
the strap hanging out. I saw a student that took off with the entire
passenger seat buckle hanging out the passenger door. That metal
buckle banging against the fuselage rattled him enough that he made an
immediate off-airport landing in a field just past the end of the
runway. Fortunately for him, he didn't damage the airplane in the
landing and it just had to be towed back to the airport. I was one of
the first ones on the scene and when he described the noise, I told him
to walk around and check the passenger door. I'll bet it took him a
long time to live that one down :-)

John Galban=====N4BQ (PA28-180)