About after a minute of starting my car, I hear a really loud beep (sounds like a call waiting beep a beep over an intercom system). The sound is coming from my Trailblazer somewhere between the driver's side stereo speaker and the steering wheel. Any ideas on what this might be??

It sounds like you may be hearing a noise coming from your vehicle's locking system, which is likely the Locktronics box. It is possible that the noise you are hearing is a normal function of the Locktronics system, especially if it sounds like a beeping or clicking noise.
Locktronics systems use a series of solenoids and relays to lock and unlock the doors, and this process can sometimes create a clicking or beeping noise. This noise can be more pronounced in certain vehicle models or aftermarket installations.
However, it is also possible that the noise you are hearing is caused by an issue with the Locktronics system, such as a malfunctioning relay or solenoid. If the noise is accompanied by other symptoms, such as difficulty locking or unlocking the doors, or if the noise is very loud and persistent, it may be best to have the system checked by a professional.
If the noise is coming from under the dash, it could be the actuator motor, which is responsible for the lock and unlock function, making noise. It's best to diagnose the problem by a professional or contact the manufacturer for assistance.

check and test relays you really dont hear an immobilizer kicking out most likely a relay overheating --also scan vehicle for code[s]

could it be overheating and what you are hearing is the radiator cap blowing off steam

can't help until listen, load the audio

sounds like you need to replace the front wheel bearings, very common problem.

not enuf fluid.

Too many possible causes for this. Take it into the dealership and have it properly diagnosed. Sounds to me to be a BCM problem (body control module).

You need to take the car back to the place that did the exhaust work. What you are hearing is the exhaust system (pipe, muffler, etc.) touching the underside of the car. The exhaust system is mounted along the underside of the car with exhaust hangers (usually made of rubber) also called insulators. The reason you don't hear it when going is because when you step on the gas the engine torques causing the exhaust system to move away from what it is touching.

This seems to be a classic case of "disentegrating exhaust system syndrome." There is either a hole(s) somewhere in your exhaust system or there is a leak somewhere. Probably the easiest way to tell where the noise is coming from is to start your car and just listen around in the engine bay, under the car, and at the muffler. Also do a visual inspection of the exhaust system to see if you can see any holes in the piping. I have also seen hairline cracks in the exhaust header itself that can cause the loud ticking noise that can be a little tricky to track down.

>  I heard a loud knocking sound when i turned the ignition
This may have been there a long time but you are only hearing it now because you are anticipating a problem. If it is only at the very first, it could be either the solenoid you are hearing or simply the teeth of the starter and flywheel not meshing quietly.
If it seems to be cranking too long, I'd check the fuel pressure up front; there is normally a capped valve that when exposed looks a lot like a tire valve. If you let it crank briefly  (maybe 2 seconds) and then poke the valve to open it, you should have pressure there if the system isn't leaking somewhere and losing pressure. If it is, you are waiting for fuel to arrive.
Long cranking is most often caused by a wait for fuel to arrive from the gas tank.