Why is my freewheel slipping?
Since this was mentioned in a comment, freewheels are not the same as freehubs+cassettes (see http://sheldonbrown.com/free-k7.html) for details. The rest of the discussion should hold if freewheel is replaced with cassette+freehub though with minor modifications - I don't know if this bike has a freewheel or a cassette+freehub since I can't load the product page. Freewheels and chains (which needs to be measured with a ruler, due to some issues with most chain checking tools according to Sheldon Brown), (and less frequently, the chain rings) are wear items - they don't last forever (and the ratcheting mechanism can go and make a lot of noise). In 3 years, you may have exceeded it in commuting depending on weather and mileage - Last freewheel I had lasted about 3500-5000 miles (some cheap sunrace freewheel) since I ran a while with a worn chain. You don't fix a freewheel unless you have to - theres a lot of bearings and what not which makes the complexity high. Buy a replacement freewheel and have it installed, and throw the old one away (and usually, thats a good time to replace your chain as well, though you should get at least 2-3 chains / freewheel). 

I wouldn't call it a banging noise (I haven't thought of what a good description of what the noise would be), but hubs which need repacking or axle damage also do make noise, so you may want to check that too. You can also get a banging-clunking noise from a failing bottom bracket as well (which can lead to a slipping sensation), so checking the bottom bracket area is another thing to do (in doing this, you'll also have to check the cranks and pedals, so this is a bit of work).