Cannondaleriders

I took my air filter out to examine it and notice a hole behind it. The hole seems to be leaking oil. When I replaced the air filter and rode it a bit, I notice oil leaking from the air filter housing. I opened it again and removed the air filter and notice that the hole was leaking more oil. Before I removed that air filter the first time, it was not leaking as much as it is now. Is this suppose to happen?

are you talking about a hole in the airbox? If your overfill your oil, this will sometimes cause oil to be puked into the airbox

Will that cause any problems?

How do I clean the airbox drain tube?

Well I think you already had the problem, which was the oil puking into the airbox, unless someone else thinks otherwise? Just be careful from now on to read the manual and add only the specified amount, there is a small rubber bubble/cup capping off your airbox drain, the drain itself is very small. It is held on with a clamp spring. You should be able to see it if you look up under your air box, its pretty small but if you get at it right you should be able to get it off without too much trouble, and then just wipe it down real good with a rag or whatever. When I took mine off I made a small slit in the bottom of it so that I could drain the water out of the box without having to fight with getting that rubber cap off every time. It's kind of like the mouth piece on a camel back, it is pretty much sealed with a slit across it, all you have to do is apply pressure on the side and the slit will open up and drain. Just my .02 hope it helps.