Diagnose this problem:
 

Went out for a ride today, but I noticed that there was a really bad rattlign noise- worst when I went over bumps. Stopped a couple of times to check, couldn't work out what it was- bumped it about on the stand and nothing. Carried on, clatter clatter. Starting to get a bit paranoid, I've been tinkering these last few days and it's been known for me to leave a bit off. New brake disc just fitted, serviced the caliper, lots of things to go catatrophicalyl wrong there. Check the wheel. All present and correct. Brakes fine too. Clatter clatter.

Who can guess what it was?

Front mud guard come loose? Happened to me once when i didn't tighten it up properly after refitting!!

Or maybe tools left under your seat?

Horn?

Screw driver/spanner that has slipped down under the seat somewhere?

You're all giving me just a bit too much credit here...

keys in your pocket!

Closer...

change in ya jacket? or tools?

You were so close too. I'd attached a bloody great padlock to my luggage rack, and it was rattling off the frame.