What could be the knocking noise coming from the engine area?

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.

Sue. I'm going to give my two cents worth because you're getting a little Miss information here from too many different directions. the vehicle in question has a long history and track record the knock noise you're describing. General Motors changed their formulation of coolant to something called dex-cool. now there was a lot of struggle within GM with dex-cool and it took them a good 15 years to get it right. what you're describing sounds to me like an example of that struggle. the Dexcool tends to be very corrosive to the intake manifold gaskets and the silicon used to seal them. coupled with General Motors recommendations of not changing the coolant for a hundred thousand miles to 150,000 miles leftmost General Motors cars being severely neglected in the cooling system department. estimate of time between service intervals was way too far. by your initial description and it going off your initial description only I personally would wager that your problem is with the dex-cool and the intake manifold gasket. the reason I say that is I live in a very mild climate it isn't so hard on cooling systems and as a 20-year ASE Master automotive tech I have done hundreds of these darn things. it seems like more than 80% of these motors off fail for the same reason. that reason is contamination of coolant into the oiling system. if you seem to be losing coolant and you don't know where it's going mostly cuz its leaking internally in the engine. even a very small amount mixed with the oil will strip the rod and main bearings and rocker arm shafts of the protective coating. if you are hearing the noise 9 times outta 10 the problem is already too large to deal with anybody needs a replacement. it is my experience once you start having that noise problem the problem is systemic throughout the system. now if its in the upper valvetrain that you're hearing the noise first in the rocker shafts. Doyle that's in the rocker shaft first started in the oil pan and the crankshaft and the rod bearings main bearings and work is way up through the motor in the passageways up to the top where it then return through our passageways back to the bottom of the engine. so if there's noise up there there's contamination likely up there if there contamination up there it's likely the contamination is systemic. you can get away with doing about job placing rocker shaft that kind of stuff but its only going to be a bandaid fix. sooner you can get the car or better yet have it towed to a prare the better I mean even if it's sitting there overnight not running that mix of coolant is catching away at theI soon are the better I mean even if it's sitting there overnight not running that mix of coolant is etching away at the bearings. so I stress the sooner the better. I have seen him and survived 4. Time afterwards but lately you've taken a good chunk off into the life of the engine. so even if the outcome is good give me want to consider trading it in. sorry for the bad news but I've seen many many many of these. Like the others have said there are other possibilities they just aren't real pattern failures on that particular engine the coolant and oil mix through the intake manifold is definitely the highest. I'm by your description I would almost bet money that that's the problem. but being as I am a mechanic and not a gambler do not Gamble other people's money nor do I gamble with mine. I get the facts and make an informed decision. that is the course of action I recommend for you. Dollar General Motors dealership will know this problem and know the vehicle well if you have a trusted mechanic by all means bring it to him or her. if you do not the General Motors dealer is a good place to bring it to be checked out they will be able to give you a good diagnosis. I hope the best. good luck Sue

Bad bearing, lifter,push rod ,flywheel, piston skirt, timing ,piston,distributor,number of things really

Maybe your EGR valve is stuck closed? Do you notice any loss of power, hesitation or lack of acceleration at all?

My answer is pull a plug passenger side front or rear these engines are bad about leaking head gaskets, water in cylinder will usually show the problem, I have replaced many.

check your radiator cap

check your egr valve its gunked up.