Gaffer tape, a good one for biking toolkit! 50 miles thinking, "where the heck is that rattling noise coming from?"
Suspecting the worst, gearbox dragging on road, con rod rubbing on rear wheel, piston smashing into handlebars etc. A gail force petrol shower to the chest suddenly gives the answer. A loose petrol cap, as usual the answer is right there in front of the ole eyes.
Trusty gaffa to the rescue (and a snapped pen).