2011 Prius - Buzzing sound during warm up in ICE on HSI.

My sister bought a 2013 Prius C and was nice enough to give me her 2011 Prius for my birthday. I went from my 2003 Camry V6 getting 23 mpg to 58 mpg on the Prius. I really enjoy the car and it saves me around $100 in gas a month.

I want to know if this sound is normal. When the radio is off, I can hear a faint buzzing or rattling sound during the warm up period. This happens in the ICE of HSI and the sound stops when I take foot off gas. The sound is mostly gone after 5 minutes of driving.

I recorded an example in youtube but it won't let me post link.
watch?v=QSgnMN7GyZk
Add youtube in front with /

Thanks.