What The Heck Is That Noise!?!

I flew over to Apple Valley (APV) to put in a little cross country time
with a GPS MAP 196 in a Cessna 150 - and to indulge in a $150 diet-Pepsi.
This particular airplane is the only available rental at my home patch at
IYK, which we locals lovingly refer to as Inyokern International. (If you
have a couple airplanes and want to be your own boss, there's an empty FBO
facility there begging for occupation. I'm sure the airport management
district will give you a good deal. But that's, as they say, another story.
Back to Apple Valley.
After dumping my waste water, replacing it with the diet soda, and
getting the kinks out of my knee and back, I wandered back to the airplane.
It had been in plain sight all the time, just across the tarmac from a pilot
friendly little cafe. So I did a cursory walk-around, wiggling a few things
here and there and checked the oil. The trip over only burned an hour's fuel
out of full tanks so I was good to go.
Mind you, I have less than ten hours in this airplane so I'm not
intimately familiar with all its quirks. I know it is well maintained and
the owner is the CFI that brought me back into flying after a 25-year
layoff. This morning, the airmass over the Antelope Valley was rock solid.
My airspeed was within two knots going and coming. The airplane trimmed out
and flew with only an occasional nudge to keep the altitude within ±40 feet.
My only gripe is an out of trim condition that requires a tiny bit of
pressure on the right rudder pedal to keep the ball in the cage.
The engine started without priming. I taxied to the end of R-18, the
favored calm wind runway, did the complete "Before Takeoff" checklist and
waited for Cessna in front of me to depart. When they lifted off. I
announced my intentions (non-towered field) rolled onto the runway, shoved
the throttle in and started the takeoff roll.
The owner likes us to hold a little back pressure on the takeoff to ease
wear and tear on the nose gear, so it takes a bit longer to gather speed and
lift off. No sweat, APV R-18 is 6,498 feet.
The plane lifted off just as it had before, then....
WHAT THE HECK IS THAT NOISE!?!
Something was rattling and thumping hard enough and loud enough to
be heard over the muffling of my passive headset. It sounded mechanical, but
didn't ding like metal. Engine was smooth. Carb heat was off. Controls felt
fine. Hmmm, did I screw the dip-stick in after I checked the oil? Oil
pressure was good. No streaks on the side windows.
Actually, I'd had an inkling something amiss about halfway through
the takeoff roll, but it didn't register as anything serious. I dropped the
nose, ready to cut the throttle and land straight ahead but didn't like the
perspective. I rejected the notion to land on the crossing runway at the
end. My choice was a shallow left turn at 500 AGL into a close-aboard
downwind. I pulled full carb heat and brought the power back to hold 80K and
altitude.
The Cessna that departed in front of me stayed in the pattern for a
normal downwind to shoot T&Gs. I called on the CTAF and told him I had a
sick airplane and asked him to extend downwind and let me have the runway.
He graciously obliged, as did the airplane ready to take the runway. Just
past midfield, I dropped ten-degrees of flaps. No change in the rattle,
thump.
I did a shallow 180 just past the approach end, feeling confident that I
had plenty of energy in case the fan should stop spinning. Lined up, I
dropped another 10 degrees of flaps. The landing was a kisser. I should do
that every time. I turned off the runway at the first available
intersection. By that time I had the carb-heat off but didn't mess with the
flaps. Instead of turning onto the taxiway, I drove straight ahead into a
resident hangar area. While the plane was still rolling, I pulled the
moisture to lean-off, switched off the mags, and opened the master switch.
Here's the embarrassing part. The noise? When I'd shut the door, I'd
left the tail end of the seat belt strap hanging outside-just about two-,
maybe three-inches of it. It was apparent, because the door was wedged shut
and required a bit of shoulder pressure to open.
That didn't stop me from going over the airplane with a preflight as
extensive as I'd do knowing that someone else had flown it.
Thirty minutes later, with a rehydrated mouth and the puckers
smoothed out of the other orifice, I took off again. As soon and the plane
was off the ground, I dropped the nose and let it accelerate flying just a
few feet off the surface, imminently ready to cut power and land if the
noise came back.
The flight home was great. I got to watch Torch-27 shoot an ILS
approach into Palmdale. Torch-27 is an F-117. I learned enough about the GPS
MAP 196 to be confident of using it cross-country.
...and I added one tiny detail to the Before Taxi checklist.