overheating problem

Hi guys -

I'm having trouble with my 2001 A4 2.8 overheating. The other day I notice my coolant was low so I added 50/50 to the fill line. The next day when leaving work I noticed when I turned it on there was a horrible knocking sound coming from under the hood I took a look and it was dark out so I didnt really see anything. Home was only 5 mins away so I drove home and by the time I got home the sound was gone. The next morning I went out to start it up and the knocking was back (seems to only happen when the car is cold). I drove again after the car warmed up and the knocking sound was gone...after about 5 mins my temp gauge was on HOT. I pulled over and waited until it cooled down so that I could drive it back home.

Any ideas? I assume its either thermo or water pump but your input would be much appreciated.

Car details: 2001 Audi A4 104,000 miles. ( I haven't had timing belt service yet)