Originally Posted by Llare

You have air in your coolant lines you need to bleed them properly...

Llare has spoken, problem solved

***Edit***

Let me explain a little better. You have air in your system from either removing the coolant at some point or perhaps from the factory (it happens). What you are hearing is air bubbles running through the coolant lines, this is giving you the "choo choo" party train noise. The reason you only hear this when the car is heating up is because the coolant does not cycle till the motor is hot.

Next... the reason your heater is cooling is because the air bubbles in the system are causing pockets that get trapped in the heater core which in turn does not allow you to get heat into the cabin.

PM if you would like me to fix this problem at my place. I have worked on this problem a lot.