Why is the heater not blowing air when heat turned on?

What is your question?

A couple of weeks ago I heard a sound when heat was turned on, it sounded like something was rattling in vents; hot air blew out of the vents. A few days ago when I turned on the heat nothing came out of the vents. I switched to the AC and it started blowing cold air, I immediately switched it back to heat and hot air was blowing from the vents. Any suggestions as to what this could be?? Any help will be greatly appreciated.

What makes this problem better or worse?

Heater blows hot air if AC is turned on first and then switched back to heat.

How long has your 2011 BMW 328i xDrive had this problem?

A few weeks