Rumbling at Idle, Cold

[Matt]

Up here in Canada it's starting to get cold again... I usually drive my car
to school in the morning, and have noticed that when I stop at the first
light on my way, and the engine is still cold, that sometimes there is a
slight rumbling, similar to what happened on our '94 Explorer when a vaccuum
hose got disconnected. It's on a much smaller scale, and I haven't had any
other troubles with the car, but I was just wondering if it's something I
should look into? Could it just be since the temps are beginning to drop
below zero overnight? BTW that first light is usually one of the only stops
on my way to school, and I don't think it happens at any other time.

Thanks,

Matt

'02 Sentra SE-R