The system becomes contaminated
Without seeing or pressure testing the system my most educated guess would be that the intake manifold is leaking.
Because it's leaking somewhere. Other than lines in the cooling system, head and intake manifold gaskets are a possibility.
Check to see if there is coolant on the engine block. It is common for the Monte Carlos, along with other GM cars equipped with the 3400 V6 engine to experience leaking intake manifold gaskets. A leaking intake manifold will cause oil to leak into the coolant system and even worse, for coolant to leak into the oil system.
Are you referring to a loud exhaust sound? If so, it could be (Going from the engine back to the tailpipe) a cracked or leaking exhaust manifold - a leaking exhaust pipe - a leaking catalytic converter - a leaking muffler. That covers the exhaust system. If you are talking about a noisy clattering sound in the engine, you could possibly need a valve adjustment.
A dehumidifier
It could just be condensation which forms in the exhaust system outside of the engine, or you could be leaking coolant into your engine (typically via the head or intake manifold gasket).
Freon could be leaking anywhere in the ac system. A colored dye can be added to the system, then seen with a special light to find the leak.
Off the manifold.
Anytime you open the system to the atmosphere you have to evacuate the system to remove contaminates and moisture.
Typically, if your car is leaking water it has to do with the air conditioning system. This simply usually means the condensation is leaking from the AC.
Remove toxic
That could be a leaking exhaust system. A leaking exhaust system is very dangerous! Please have it checked out ASAP.