99 A6 2.8 quattro with 157K

pretty sure its when the heat was on.....it happened twice today ok so i heard a loud groaning noise from the front of my car.. and when i turned the heat off it was gone.. what is it ? and when the heats off it doesnt happen ... some one told me one of my fans was getting stuck? i dunno know...someone help

thanks

 

check the fan clutch and serpentine belt loose.

 

any noise when the AC is on? I had a noise issue with the blower- When hot air was blowing it made a horrible sound, but not when the AC was on- I replaced my HVAC blower and all is quiet-

 

ok where would i find the HVAC ? and how much was it to get replaced and it doesnt do it with the AC on ..just heat ..but now yesturday it didnt do it at all and today it hasnt ...its just like one day out of the week that it decides to do it and its only for like a minute.