This is one of those hard to diagnose problems because I don't have the car in front of me. F it was just a radiator leaking, and you just ran it too low, adding coolant will presumably fix the problem until it runs low again. I do not like the clunk, but more than that I do not like the smoke from the tailpipe.
It could be that you are in the downward spiral of a blown head gasket. When your head gasket starts to go, it pressurizes the coolant chambers and can make the radiator look like it is leaking (and it can cause a real leak too). You will start to loose coolant out of the tail pipe, and smell it too. If you didn't mention the tail pipe it wouldn't be so bad, but either way overheating regardless of the cause can lead to, if not create the issue, of a blown head gasket.
There is one product that I have seen fix two cars so far, and for $30 is worth trying. It's a Bars Leak head gasket repair in a bottle. Not long ago I would say it's a waste of money, but like I said, twice I have seen it work with my own eyes. One of those times however, the car was driven on too long with the bad head gasket and it did fix the head gasket but rod bearing damage was done to the motor which resulted in low oil pressure. If you get lucky and din't do further damage, this may solve your problem for a while.
Oh, and in the other case that I saw it, it was used to get a car off the highway which had a known head gasket leak (20 psi in the #1 cylinder)and when it came to me all cylinders held 120 psi with my leak down tester. The car is still driving today. Good luck, if you decide to buy it you can get it at autozone, advance and pepboys. Make sure it's the one where you just add it to the coolant without a drain and flush.
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Wednesday, January 18th, 2012 AT 7:36 PM