If you hear the noise for just a few seconds it may be the ABS self check...

 

so I took my car to the dealership yesterday, I wasn't be able to replicate the noise from the engine cuz it was sort of a random event, plus the the fact that it only happened when the car is cold, well, it took me about 30 mins to drive to the dealership.. so yeah.
As for the periodic noise on the back wheels, the guy was saying that it's b/c the brake pad is hard when car is cold, when car warms up and drive for a bit, the noise disappears (again, another cold start issue)... to say the least, I am still a bit skeptical, does that brake pad story make sense though?