Clicking noise when i start my car there is a loud clicking noise coming from my engine. It sounds like it is coming from my oil pan. What could it be

I would remove or have your oil pan removed and check for any unwanted pieces in there. Maybe first see if there is anything stuck to drain screw. Drain oil maybe... You may have blown a rod.

This noise can be caused by a defective outboard (wheel end) front axle constant velocity joint (known as a "CV" Joint). This would be louder on turns, more noticeable at lower speeds and sharp turns. Next it could be valve train noise, this would come from the top of the engine and this could be related to the oil change if a piece of dirt got into the oil supply to the hydraulic camshaft followers. At this point you need a professional to listen to the noise to determine the exact cause.

the knock sensor changes the timing of the motor when it senses knock ,it sounds like the wire may have come off ,it sounds as if when you press the gas the timing should advance and it sounds as if its not advancing thats why you are hearing a clattering noise on accelerating.if you keep driving it like that you can do engine damage

Sounds potentially terminal. Check your oil level on the dip stick. Do not attempt to drive the vehicle if you suspect loss of oil, best to have it towed in neutral. Check that the engine is first OK: drain out the oil and remove the pan/sump from the bottom of the engine. Check pan interior for any debris. Look at crank, connecting rods, big end and main bearing shells.

sounds like coolant has leaked into the oil pan, loosen the drain plug slowly , if water comes out before the oil the engine is probably badly damaged already, teardown and inspection comes next

sounds like it just needs a oil & filter change ?
check engine oil level!
smell it to see if its burnt
or is dirty [tarnished or black]
or gritty between your fingers?
it could be noisy tappets or lifters!
change your mechanic as its got absolutely nothing to do with the belts!
im truly amazed at the amount of rip offs in the auto industry in the US!
Cheers Rob.

It sounds like one of your pistons are bad as in the connecting rod pin's to the piston is loose and your getting to much internal engine pressure, that's why your getting lot's of oil coming out the top if your engine. Sorry for the news friend but time to retire the car, the cost is going to be greater to repair then replace. 

I'd consider some fresh, clean oil & filter change first of all, if it hasn't been done . drive it for a while providing it isn't too bad, & see if it goes away, if not get back to us.

If you heard a loud knocking.....I'm sorry to say, I'd hope for the worst. It doesn't sound good that you lost that much oil.

But try to clean it with a steamer or with some brake clean. Put a little oil back in and see where it is coming from. If it runs out without starting the car.....then you have definitely cracked the block, or the oil pan.

low oil psi need to remove pan and check oil pickup