Question about temp.

[Cutie Pie #2]

It has been very cold here and probably everywhere else, well my concern is I have noticed that the scan gauge temp. has been going up and down every time I let off the gas by about 20-30 degrees, also when it does that the temp. coming from heater goes up and down also. I also have notice in the morning when first start the car there is a noise for a second or two (sounds like a car without a muffler) that has only happen 2 times. Could the water pump be going bad or is this just a regular thing with the Yaris. All is fine after that except for what was mention above. I'm taking car in to dealer to get oil change and will ask them to check it out. Just wanted to know if anyone else has had this happen?