heater issue?

[poppaj3]

07 el eddiebaur
found new issue
when we adjust the temp, when we hit 78-79 there is a plasitic like groan coming from behind the dash, 76, 77 nothing, 80, 81 nothing,
just those two temps. ok, so dont use those 2 temps, but it seems very strange, any had a similar issue, or expert thought? Thanks.

 

[ELVATO]

It does it regardless of outside temp? Probably just the blend doors trying to mix hot and cold.