It's just getting cold here in PA, and I just started using the heat this week. I noticed that when I start my car up when it's cold, there is a slight bubbling sound coming from the dash. If I rev the engine a little bit the bubbling seems to go away. I'm assuming this is air somewhere in the heater core. Is this a problem? Will it do damage? And do I have to flush the whole system, or is there a better way to get rid of the bubbles?