Insofar as your leak goes obviously check your hoses for loose clamps any hoses that are dried out/cracking if they aren't leaking now they soon will be and the radiator. One thing many people overlook when looking for the source of coolant leaks is the water pump. It has a weep hole and when the pump bearings are shot it will begin to leak to let people know to look for a problem. If you have looked and looked and not found the leak. It is probably from the water pump. It starts out dripping slowly as the seal wears from the play in bearing's movement. Until the pump is really ready to go it only drips out when the engine is running.
You have not given much to go on to reasonably ascertain what would cause the sound you mention. Does it happen when you are stopped and the engine is idling? When stopped and pressing on the gas does it increase, slow down or stop? Have you had exhaust work done lately? Many times people don't run the pipes properly and they will thump against something as you describe. I would check your engine and motor mounts if one of them has been compromised it can give the sound you describe while the car is idling. If it only happens when you are going down the road everything I've mentioned so far and a whole lot more could be causing it. From what I have said, oldturkey03 stated and your own experience it is going to be somewhat hit and miss diagnosing the cause(s). If it is coming from the motor, a sensor is more than likely picking it up and in part defining the cause. I would suggest hooking it up to a diagnostic scan tester to help determine the cause. A number of people are not aware of this but many auto parts stores, including AutoZone, offer free diagnostic hook-ups. I would call around to find an auto parts store that does the free diagnostic scan on your car's computer. While they have it hooked up casually ask them about the noise and possible causes. Most of the counter people have a fair amount of experience working on cars and may very well know the cause. This is not a direct solution to the problem. But, it is kind of hard not being able to hear the noise or knowing when it is happening to give you any reliable advice besides take it to someone that can hear it and determine when it is happening. While this is not a direct solution to your problem it is the most economical and feasible one I can think of.
NOTE: The computer diagnostic scanners that the auto parts store will hook up for free is the same one(s) used by many mechanics who will charge you for the service.