Loud knocking when heating is turned up on Audi B8

[GazzaA4B8]

Hi
I appologise if I have posted in the wrong place, but i'm new....

I have a question to all?

My Audi A4 B8 is driving me mad.
When the heating is off, the car runs really well, but when I turn the heating on, its like something is trying to smash its way through the back of the entertainment system.

Really strange, I am going to book it in for a proffesional to look at it (Im not mechanically minded).

Is there an easy solution or will I have to take out a loan?

Thanks in advance...