Bike noises

It's a beautiful day so I thought I'd go for a bike ride and up my Vitamin D levels. First the bike needed a bit of cleaning and tinkering and whilst wasting time prior to actually doing anything I came across this excellent looking site http://www.jimlangley.net/wrench/keepitquiet.html Many of the irritating noises your bike makes are documented here with the solutions. Unfortunately I still can't quite work out what the rattling noise is from my back wheel. Sounds like a ball bearing rattling around but is that really what it could be? Might need an expensive trip to the bike shop. ho hum...