Heating 230-4 Interior
 

Gentlemen
Cold weather has arrived, it actually snowed this morning. urgent!!
I have hot engine, defrost fan works, but unable to get heat using fan. Tried A/C and fan volume is great. When I shut A/C down you can hear trap inside ducting either opening or closing.
presume that this is not happening when asking for defroster heat.

Can anyone enlighten me as to what I am looking for to correct problem??