Back seat noises

For the last couple of weeks I'd been hearing a little noise coming from near the back seat of my car when I went around corners or over bumps - a little click. It was driving me a little mad. So last week I unclipped the rear seat, and poked around. Right next to the seatbelt latch I found a stray wheel nut. I sighed with relief!

The sound was back the next day. Again after a day, I found time to dig around, this time with seats folded down. I found another wheel nut. Also my car's towing ring. And also an ancient Nokia phone charger. 

Bizarre...