Car has been in the upper midwest,(college) the past 2 years. Winter temps as low as -25 F. No problems with interior heat at all. Believe me, I'd hear about it if there was. Flush/fill was performed prior to winter 2017/2018, including the heater core of course. Heater function was no issue. However, the overflow tank level is lower than it should be (engine is currently hot). I'm letting it cool down (takes a while here in AZ) then I'll try bleeding.
Don't know why the sounds are only heard upon harder acceleration though. Quiet during normal city driving.