Oscillating idle / shudders with HEAT ON - 09 hatchback

[James in Denver]

Howdy Yaris experts,

My first post :)

I have a Bayou Blue 2009 Yaris Hatchback 5 speed. Just got it 3 months ago used, but with only 8500 miles.

Anyway, its gotten colder in Colorado and I've noticed that the car idles very roughly when I turn on the heat. It oscillates from normal say 5 secs to vibrating the front of the car for maybe 1 sec, then back to normal. This happens when its warmed up usually.

Outside temp is 30s to 50s, car temp light is off (although it might happen when its still cold, never checked).

It idles normal when the heat is off. Haven't tried the AC yet as its cold outside. No difference with headlights on or off so I don't think its power/alternator related.

I have the remaining factory warranty, but I want at least a little idea of what might be wrong so I don't get screwed by the service techs.

Any ideas?

James in Denver