I removed my old water heater..,?

yes, there's air in the newly installed system -- you have to let it out (open the hot water taps in bathroom) and the power should be OFF while you do that

Just to consolidate, there's air in the line and what you hear is it escaping. Quite normal. Run the water until it stops gurgling, belching and hissing and when it is running clear and steady THEN you turn on the heat. Not before.

Yes, til the air clears the line.

Unless that is gas heating in which case you should definately call a professional.

rnaybe you should get a rnechanic to take a look at it

Not that weird for a water heater. They are big players. I think you are supposed to turn on the water before you turn on the electric power. I think it may automatically not turn on with no water. If it does come on that would be not good. If you water is flowing through it, it is okay. Fuggeddaboutit the scum you see. Now you know what you are drinking every day.

You are supposed to let it run five or ten minutes to clean itself out after an operation.

Not weird at all. There is air in the line. You should have turned on the hot water valves in every bathroom and the kitchen while the water heater was filling with water. Once you saw a stream of water with no air then shut off all valves.

Normal. GIve it some time. Air in the line just needs to be replaced with water. It should go away after a few minutes.

the hissing MIGHT mean the heating elements are ON while you fill-it, not supposed to be.